Reduct

In universal algebra and in model theory, a reduct of an algebraic structure is obtained by omitting some of the operations and relations of that structure. The converse of "reduct" is "expansion."
Definition.
Let "A" be an algebraic structure (in the sense of universal algebra) or a structure in the sense of model theory, organized as a set "X" together with an indexed family of operations and relations φi on that set, with index set "I". Then the reduct of "A" defined by a subset "J" of "I" is the structure consisting of the set "X" and "J"-indexed family of operations and relations whose "j"-th operation or relation for "j"∈"J" is the "j"-th operation or relation of "A". That is, this reduct is the structure "A" with the omission of those operations and relations φ"i" for which "i" is not in "J".
A structure "A" is an expansion of "B" just when "B" is a reduct of "A". That is, reduct and expansion are mutual converses.
Examples.
The monoid (Z, +, 0) of integers under addition is a reduct of the group (Z, +, −, 0) of integers under addition and negation, obtained by omitting negation. By contrast, the monoid (N,+,0) of natural numbers under addition is not the reduct of any group.
Conversely the group (Z, +, −, 0) is the expansion of the monoid (Z, +, 0), expanding it with the operation of negation.