sperm donor

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A man who donates sperm, usually anonymously, to a sperm bank or fertility clinic.
• noun US, slang, derogatory A man who is the father of a woman's child(ren), but no longer has a relationship (or has a hostile relationship) with the woman, and is typically uninvolved with the children as well. (Compare with baby mama).