lattice theory
Join and meet – two related operations on a poset in order theory Ideal – special kind of lower sets of an order and filter (dual notions) Skew lattice – Algebraic Structure (generalization to non-commutative join and meet) Post's lattice – lattice of all clones (sets of logical connectives closed under composition and containing all projections) on a two-element set {0, 1}, ordered by inclusion Tamari lattice – mathematical object formed by an order on the way of parenthesing an expression