Browser Object Model
一言で言うと：JavaScriptがbrowserとおしゃべりできるようになるためのオブジェクト もうちょい詳しく
Though ECMAScript describes it as the core of JavaScript, the Browser Object Model (BOM) is really the core of using JavaScript on the Web. The BOM provides objects that expose browser functionality independent of any web page content.
For years, a lack of any real specification made the BOM both interesting and problematic, because browser vendors were free to augment it as they saw fit. The commonalities between browsers became de facto standards that have survived browser development mostly for the purpose of interoperability. Part of the HTML5 specification now covers the major aspects of the BOM, as the W3C seeks to standardize one of the most fundamental parts of JavaScript in the browser.
具体的には
At the core of the BOM is the window object, which represents an instance of the browser. The window object serves a dual purpose in browsers, acting as the JavaScript interface to the browser window and the ECMAScript Global object. This means that every object, variable, and function defined in a web page uses window as its Global object and has access to methods like parseInt().
BOMを構成する要素の一つであるdocumentノードはDOMと呼ばれる