Definition
A man who is the father of a woman's child or children, but no longer has a relationship, or has a hostile relationship, with the woman, and is typically uninvolved with the children as well.
How People Actually Use It
The person that gets a female pregnant but doesn't help her with the child in any way, shape, or form. In some cases they don't acknowledge the child at all.
"I saw my sperm donor today. He came by to see my daughter and had the nerve to be empty handed."
Source: Urban Dictionary