If a custodial parent is living by his/herself with the children, would you consider that person a single parent if the other parent is very active and looks after the children on a regular basis?

I have often wondered if the moniker “single parent” is only given to parents who are unmarried and living alone, regardless of how active the other parent is in looking after and raising the child(ren). What about situations where, say, both parents live close enough to each other for the child(ren) to be able to go to either house after school and the non-custodial parent looks after the kid(s) three or four times a week. Would you consider the custodial parent a single parent in that situation?