ActionScript is an object-oriented language originally developed by Macromedia (now owned by Adobe Systems) for its authoring tool. It's a dialect of ECMAScript that supersets the syntax and semantics of JavaScript.
The first three versions of the authoring tool provided limited interactivity features. Early developers could attach a simple command, called an "action", to a button or a frame. The set of actions was basic navigation controls, with commands such as "play", "stop", "getURL", and "gotoAndPlay".
With the release of Flash 4 in 1999, this simple set of actions became a small scripting language with many new capabilities that include variables, expressions, operators, if statements, and loops.
After Macromedia was acquired by Adobe, it's now developed as Adobe Flash to embed SWF files to web pages.