A level of classification in the taxonomic system, a family is a group of similar genera, and related families are grouped together in the same order. Classification is based on levels of similarities; organisms with a broad outline of similar characteristics are placed in the same family. Families can vary in size; for example, the orchidaceae contains all members of the orchids, some 18 000 species contained in 750 genera.

Within botany, all family names end in -aceae. In zoology, the family names end in -idea. There is no specific fixed number of characteristics which must be similar to group organisms into a family. The division into this group is artificial and must be decided by a taxonomist who is an expert in the group under discussion. Over time proscribing characteristics have been defined for most families, so the presence of those characteristics instantly identifies the organism under question to belong to that family. A family is not necessarily as apparent as the similarities that are present at the generic level, or indeed at some of the higher levels of taxonomy.