A brother is a man or boy who shares one or more parents with another; a male sibling. The female counterpart is a sister. Although the term typically refers to a familial relationship, it is sometimes used endearingly to refer to non-familial relationships. A full brother is a first degree relative.

A sibling is a relative that shares at least one parent with the other person. A male sibling is a brother, and a female sibling is a sister. Somebody with no siblings is an only child.