http://www.perlmonks.org?node_id=466496


in reply to What are "the odds"?
in thread Fastest way to calculate hypergeometric distribution probabilities (i.e. BIG factorials)?

In the example you gave the odds are 3:1 (or 3, if one follows the common practice viewing this as a ratio), but decidedly not 25%. Granted, in colloquial speech, "odds" and "probability" are often used interchangeably, but technically odds and probabilities are "interchangeable" only in the limited sense that one can derive one from the other. They mean different things. See here.

Update: In response to chas's comment, I replaced the link to MathWorld's definition (which was, indeed, well, odd) with a link to a clearer explanation of odds vs. probabilities.

the lowliest monk