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.
Do not use together with the toned tag when describing one character. If the character has an athletic build that’s not overly bulky, use toned instead.
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).