domain

do·main

noun \dō-ˈmān, də-\

Definition of DOMAIN

1

: any of the three-dimensional subunits of a protein that together make up its tertiary structure, that are formed by folding its linear peptide chain, and that are variously considered to be the basic units of protein structure, function, and evolution <immunoglobulin light chains have two domains and heavy chains have four or five domains, depending on class—Journal of the American Medical Association>

2

: the highest taxonomic category in biological classification ranking above the kingdom