You know, strangely enough, back when this had an anime-specific name, it also had a description bordering on real trope, combining not only appearance but personality, typical occupations, and some of the ways the character was used. Not that the original description couldn't have been better, but now it is so generic as to be meaningless.
Most shounen series will invariably have a mother (often a Christmas Cake) who is noticeably attractive. In appearance and disposition, she is more often than not either as sweet-cute-and-huggable as her younger counterparts and/or hot enough to give Mrs Robinson a run for her money. This may even be referenced in the show by another character commenting on her "down-to-earth charm" or "earthy good looks". If she's the only recurring female adult, she'll often be popular with the older fandom. As a rule, Hilarity Ensues as she will almost certainly be mistaken for her children's cute big-sister, much to their embarrasment.
Mothers are, of course, seen in other kinds of shows, but have a special significance in shounen, where they usually reflect the relative healthiness of the household without needing to show an extended family. If they're homemakers, they'll be (conveniently) always present at home, and often in the kitchen; if they work outside the house, their jobs will either be "feminine" (nurse, school teacher, waitress, secretary, etc.) or "intelectual" (lawyer, doctor, librarian, etc). They'll be more attractive to the soon-to-be-teenage audience since the Hot Mom is a realistic way to keep the cast predominantly female.
This rule tends to hold most strongly for the lead character, while a side character is more likely to have a bombastic overbearing mother. Lead female characters, especially in Unwanted Harem series, can also have such mothers; if the lead lives with her, it's a general rule.
Occasionally the character is an aunt or big sister, but this is rare.
edited 16th Jan '12 8:14:32 PM by Catbert