Definitions for "Subfield"
A subset S of a field F is a subfield if it contains 0 and 1, and is closed under subtraction and multiplication and inverses, i.e. if x is in S and is not zero, then x-1 is in S.
A segment within a field of a bibliographic record designed for particular content. For example, the title proper is in the $a subfield of the 245 field, the other title information or subtitle is in the $b of that field, and the statement of responsiblitity is in the $c of the 245 field.
A field in a Multiple.