wtkad - 2:51 pm on Mar 12, 2007 (gmt 0)

Exact match. Or at least sticking to their own published explanation of "standard match".

We were bidding on both 'short phrase' (low bid) and 'short phrase product' (higher bid), and dropped the first because it wasn't converting well. Now our product keyword is showing up under searches for 'short phrase' (on Yahoo itself) at the higher CPC. That is not a plural or a misspelling, which are the only two additions it claims it will match for you.