'$' already defined in Chrome?

If you type $ in console of Chrome in a plain html page without script. You would see this output

> $
< bound(selector, startNode)

However when you do window.$ , the output is undefined,

> window.$
< undefined

What is the $ defined here , and why can't I access it via object window ?

$ is a local variable injected in your console from the __commandLineAPI object, via a with statement wrapping your code (that's why it's not a global variable). If you inspect it more closely (using e.g. debugger; $('*');), you'll also see that it is basically a function bound to the current window with the following source:

$: function (selector, opt_startNode) {
    if (this._canQuerySelectorOnNode(opt_startNode))
        return opt_startNode.querySelector(selector);

    return inspectedWindow.document.querySelector(selector);
}

(defined on CommandLineAPIImpl.prototype)

From Command Line API Reference: > The Command Line API is a collection of functions for performing common tasks with the Chrome Developer Tools. These include convenience functions for selecting and inspecting elements in the DOM...

> $(selector) Returns reference to the first DOM element with the specified CSS selector.This function is an alias for document.querySelector() function.

For example:

$('body')

Adding to the previous answers, it can be found in the code of VM of chrome.

// NOTE: Please keep the list of API methods below snchronized to that in WebInspector.RuntimeModel!
// NOTE: Argument names of these methods will be printed in the console, so use pretty names!
/**
 * @type {!Array.<string>}
 * @const
 */
CommandLineAPI.members_ = [
    "$", "$$", "$x", "dir", "dirxml", "keys", "values", "profile", "profileEnd",
    "monitorEvents", "unmonitorEvents", "inspect", "copy", "clear", "getEventListeners",
    "debug", "undebug", "monitor", "unmonitor", "table"
];

All the implementations can be found here

CommandLineAPIImpl.prototype = {
    /**
     * @param {string} selector
     * @param {!Node=} opt_startNode
     * @return {*}
     */
    $: function (selector, opt_startNode)
    {
        if (this._canQuerySelectorOnNode(opt_startNode))
            return opt_startNode.querySelector(selector);

        return inspectedWindow.document.querySelector(selector);
    },

    /**
     * @param {string} selector
     * @param {!Node=} opt_startNode
     * @return {*}
     */
    $$: function (selector, opt_startNode)
    {
        if (this._canQuerySelectorOnNode(opt_startNode))
            return opt_startNode.querySelectorAll(selector);
        return inspectedWindow.document.querySelectorAll(selector);
    },

    /**
     * @param {!Node=} node
     * @return {boolean}
     */
    _canQuerySelectorOnNode: function(node)
    {
        return !!node && InjectedScriptHost.subtype(node) === "node" && (node.nodeType === Node.ELEMENT_NODE || node.nodeType === Node.DOCUMENT_NODE || node.nodeType === Node.DOCUMENT_FRAGMENT_NODE);
    },

    /**
     * @param {string} xpath
     * @param {!Node=} opt_startNode
     * @return {*}
     */
    $x: function(xpath, opt_startNode)
    {
        var doc = (opt_startNode && opt_startNode.ownerDocument) || inspectedWindow.document;
        var result = doc.evaluate(xpath, opt_startNode || doc, null, XPathResult.ANY_TYPE, null);
        switch (result.resultType) {
        case XPathResult.NUMBER_TYPE:
            return result.numberValue;
        case XPathResult.STRING_TYPE:
            return result.stringValue;
        case XPathResult.BOOLEAN_TYPE:
            return result.booleanValue;
        default:
            var nodes = [];
            var node;
            while (node = result.iterateNext())
                push(nodes, node);
            return nodes;
        }
    },

    /**
     * @return {*}
     */
    dir: function(var_args)
    {
        return InjectedScriptHost.callFunction(inspectedWindow.console.dir, inspectedWindow.console, slice(arguments));
    },

    /**
     * @return {*}
     */
    dirxml: function(var_args)
    {
        return InjectedScriptHost.callFunction(inspectedWindow.console.dirxml, inspectedWindow.console, slice(arguments));
    },

    /**
     * @return {!Array.<string>}
     */
    keys: function(object)
    {
        return Object.keys(object);
    },

    /**
     * @return {!Array.<*>}
     */
    values: function(object)
    {
        var result = [];
        for (var key in object)
            push(result, object[key]);
        return result;
    },

    /**
     * @return {*}
     */
    profile: function(opt_title)
    {
        return InjectedScriptHost.callFunction(inspectedWindow.console.profile, inspectedWindow.console, slice(arguments));
    },

    /**
     * @return {*}
     */
    profileEnd: function(opt_title)
    {
        return InjectedScriptHost.callFunction(inspectedWindow.console.profileEnd, inspectedWindow.console, slice(arguments));
    },

    /**
     * @param {!Object} object
     * @param {!Array.<string>|string=} opt_types
     */
    monitorEvents: function(object, opt_types)
    {
        if (!object || !object.addEventListener || !object.removeEventListener)
            return;
        var types = this._normalizeEventTypes(opt_types);
        for (var i = 0; i < types.length; ++i) {
            object.removeEventListener(types[i], this._logEvent, false);
            object.addEventListener(types[i], this._logEvent, false);
        }
    },

    /**
     * @param {!Object} object
     * @param {!Array.<string>|string=} opt_types
     */
    unmonitorEvents: function(object, opt_types)
    {
        if (!object || !object.addEventListener || !object.removeEventListener)
            return;
        var types = this._normalizeEventTypes(opt_types);
        for (var i = 0; i < types.length; ++i)
            object.removeEventListener(types[i], this._logEvent, false);
    },

    /**
     * @param {*} object
     * @return {*}
     */
    inspect: function(object)
    {
        return injectedScript._inspect(object);
    },

    copy: function(object)
    {
        var string;
        if (injectedScript._subtype(object) === "node") {
            string = object.outerHTML;
        } else if (injectedScript.isPrimitiveValue(object)) {
            string = toString(object);
        } else {
            try {
                string = JSON.stringify(object, null, "  ");
            } catch (e) {
                string = toString(object);
            }
        }

        var hints = { copyToClipboard: true, __proto__: null };
        var remoteObject = injectedScript._wrapObject(string, "")
        InjectedScriptHost.inspect(remoteObject, hints);
    },

    clear: function()
    {
        InjectedScriptHost.clearConsoleMessages();
    },

    /**
     * @param {!Node} node
     * @return {!Array.<!{type: string, listener: function(), useCapture: boolean, remove: function()}>|undefined}
     */
    getEventListeners: function(node)
    {
        var result = nullifyObjectProto(InjectedScriptHost.getEventListeners(node));
        if (!result)
            return result;
        /** @this {{type: string, listener: function(), useCapture: boolean}} */
        var removeFunc = function()
        {
            node.removeEventListener(this.type, this.listener, this.useCapture);
        }
        for (var type in result) {
            var listeners = result[type];
            for (var i = 0, listener; listener = listeners[i]; ++i) {
                listener["type"] = type;
                listener["remove"] = removeFunc;
            }
        }
        return result;
    },

    debug: function(fn)
    {
        InjectedScriptHost.debugFunction(fn);
    },

    undebug: function(fn)
    {
        InjectedScriptHost.undebugFunction(fn);
    },

    monitor: function(fn)
    {
        InjectedScriptHost.monitorFunction(fn);
    },

    unmonitor: function(fn)
    {
        InjectedScriptHost.unmonitorFunction(fn);
    },

    table: function(data, opt_columns)
    {
        InjectedScriptHost.callFunction(inspectedWindow.console.table, inspectedWindow.console, slice(arguments));
    },

    /**
     * @param {number} num
     */
    _inspectedObject: function(num)
    {
        return InjectedScriptHost.inspectedObject(num);
    },

    /**
     * @param {!Array.<string>|string=} types
     * @return {!Array.<string>}
     */
    _normalizeEventTypes: function(types)
    {
        if (typeof types === "undefined")
            types = ["mouse", "key", "touch", "control", "load", "unload", "abort", "error", "select", "input", "change", "submit", "reset", "focus", "blur", "resize", "scroll", "search", "devicemotion", "deviceorientation"];
        else if (typeof types === "string")
            types = [types];

        var result = [];
        for (var i = 0; i < types.length; ++i) {
            if (types[i] === "mouse")
                push(result, "mousedown", "mouseup", "click", "dblclick", "mousemove", "mouseover", "mouseout", "mousewheel");
            else if (types[i] === "key")
                push(result, "keydown", "keyup", "keypress", "textInput");
            else if (types[i] === "touch")
                push(result, "touchstart", "touchmove", "touchend", "touchcancel");
            else if (types[i] === "control")
                push(result, "resize", "scroll", "zoom", "focus", "blur", "select", "input", "change", "submit", "reset");
            else
                push(result, types[i]);
        }
        return result;
    },

    /**
     * @param {!Event} event
     */
    _logEvent: function(event)
    {
        inspectedWindow.console.log(event.type, event);
    }
}

In order to check this just press bound(selector, startNode) it's an active link

It's a console only built-in function:

https://developer.chrome.com/devtools/docs/commandline-api