A branch of combinatorics that focuses on the study of words and formal languages
A word is a sequence of symbols, finite or infinite, taken from a finite alphabet. A natural environment of a finite word is a free monoid. Consequently, words can be seen as discrete combinatorial objects or discrete algebraic objects in a noncommutative structure. These two facts--–discreteness and noncommutativity–--are the two fundamental features of words. (Source)
See also: Wikipedia article Combinatorics on words.