The word gradient has many different meanings, but in CSS, a gradient is strictly defined as a gradual transition between a range of (at least two) colours. CSS gradients are a fairly recent development but have already been through a number of changes. They were originally proposed by the WebKit team in April 2008, modified from the syntax proposed for the canvas element in HTML 5.
The W3C’s CSS Working Group later proposed a modified syntax, and this syntax is in the latest revisions of the Image Values Module. The first browser to implement this was Firefox 3.6, and the WebKit team weren’t far behind, so Safari 5.1 and above and recent versions of Chrome also use this syntax (The Book of CSS3 explains the old WebKit implementation fully).