(thing) by rp Mon Apr 08 2002 at 15:00:50
In mathematics, a closure property refers to the property of a set being closed under some operation on its elements.

Some examples:

the natural numbers
are closed under addition and multiplication, but not under subtraction or division
the regular languages
are closed under intersection, union, complementation, and pairwise concatenation (X.Y = {x.y | x in X, y in Y}), but not elementwise concatenation (2X = {x.x | x in X })
mankind
is closed under multiplication, but not under its converse, unless you are a creationist

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