Noun
Catalan number (plural Catalan numbers)
English Wikipedia has an article on:Catalan numberWikipedia
In combinatorial mathematics, any of a sequence of natural numbers that occur in various counting problems, often involving recursively defined objects; the n Catalan number is equal to (2n choose n) over (n+1).
Cn — the n Catalan number — could be described as the number of distinct ways in which a sequence of n + 1 numbers could be associated by means of some specified binary operator.