Tamari lattice

Etymology
Introduced by Dov Tamari (1962).

Noun

 * 1)  A partially ordered set in which the elements consist of different ways of grouping a sequence of objects into pairs using parentheses (e.g. for a sequence of four objects abcd, the five possible groupings are ((ab)c)d, (ab)(cd), (a(bc))d, a((bc)d), and a(b(cd))).