Hasse diagram

Short definition from Wiktionary

Noun

1. 1
   A diagram which represents a finite poset, in which nodes are elements of the poset and arrows represent the order relation between elements. Transitivity of the order relation is tacit, in other words, if x<y and y<z then no arrow is drawn from x to z, but if there is no distinct z between x and y (such that x<z<y) then an arrow is draw from x to y.