A single mother is any woman that has children that she is caring for on a daily basis without help from another person. Although the husband pays child support and they have joint custody, when the children are with her, she's still considered a single mother. She's the only one caring for them at that point and time, hence the "single" part of it. The father is also considered a single father. If you really want to be technical about it, then perhaps a better "label" for her would be to refer to herself as a divorced mother. *shrug* Its just a phrase and a reference to marital status more than anything. You don't have to take it quite so literal.