What makes a single parent a single parent?

I saw a comment on my friend post from the mother of his child calling herself a single parent. Now yes they are separated and she mainly has custody as they live a ways away from each other now. But he is very active and involved in his daughters life and they didnt break up for any bad reasons it was a mutual split theres no bad blood.

So is one a single parent because they are simply single?

Should it be considered a single parent even though both parents are involved in the childs life?