The phrase "birth father" refers to the biological father of a child. Some synonyms for this term include biological father, biological parent, genetic father, and natural father. Other common terms used to refer to a birth father include father of origin, biological dad, and birth parent. These terms are typically used in the context of adoption or situations where the birth father is not actively involved in the child's life. Regardless of the term used, it is important to acknowledge the role of the birth father in a child's life and work towards establishing a positive relationship if possible.