An attractive "middle-aged" woman. Common traits include slight facewrinkles, a noticeable belly, and an overall body shape that's generally curvier than that of a younger woman. Alternate terms, depending on age range, can include cougar, hag, or MILF (Mother I'd Like to Fuck; used regardless of the fact whether she's had children or not).
A woman that is said to be a mother or shown with her own children does not automatically make her a mature female. She must look like an older woman to count.
