A female character with visibly developed muscles that must have come from extensive training, body building or otherwise.
Muscular female usually emphasize overall enlarged musculature, leaving the character looking thicker and more masculine without removing all traces of femininity. It's not just well developed abs. Look for thicker arms, legs, torsos, the whole deal.
This tag should not be used for cases where a female character just has an athletic build but is not overtly muscular, which falls under toned. Nor should it be used for female characters intentionally drawn to look like men (e.g. cho marisa with a beard, a Touhou girl drawn to look like Kenshiro).