A relative, in general; especially, a descendant, whether male or female; a granddaughter or a grandson.

A daughter of one's brother or sister, or of one's brother-in-law or sister-in-law. In modern English, this is the primary meaning.

Up until the 1600’s this meant a descendant, either male or female. Occasionally it was used to mean any younger relative.

a daughter of your brother or sister