There are a few synonyms for the term "natural father". One of the most popular synonyms is "biological father". This term refers to the father who is related to a child through genetics. Another synonym is "birth father", which is also commonly used in the context of adoption. This term acknowledges that the father is the one who fathered the child at the time of birth, but may or may not be involved in raising the child. Lastly, "sperm donor" is a term used to describe a man who provided his sperm to a woman for the purpose of reproductive assistance but may not have any involvement in raising the child. Overall, these synonyms can be used interchangeably to describe the father who is related to a child through genetics.