Operations on Languages

Languages are sets. Therefore, any operation that can be performed on sets can be performed on languages.

If L, L1 and L2 are languages, then

In addition,

L1 L2, the catenation of L1 and L2, is a language.
(The strings of L1 L2 are strings that have a word of L1 as a prefix and a word of L2 as a suffix.)
L, the catenation of L with itself n times, is a language.
L = L L LL LLL LLLL ..., the star closure of L, is a language.
L = L L LL LLL LLLL ..., the positive closure of L, is a language.