Foster brother is a term used to describe a male sibling who is raised in the same household as another child, typically not related by blood. There are several synonyms that can be used to describe a foster brother, such as "adopted brother," "stepbrother," or "brother by choice." Other phrases that may be used to describe this situation include "childhood companion" or "family friend turned brother." Regardless of the term used, the idea is to convey the close bond that develops between two individuals who are not related by blood but are raised together as brothers.