In the mathematical disciplines of in functional analysis and order theory, a Banach lattice (X,‖·‖) is a complete normed vector space with a lattice order, ≤ {\displaystyle \leq } , such that for all x, y ∈ X, the implication | x | ≤ | y | ⇒ ‖ x ‖ ≤ ‖ y ‖ {\displaystyle {|x|\leq |y|}\Rightarrow {\|x\|\leq \|y\|}} holds, where the absolute value |·| is defined as | x | = x ∨ − x := sup { x , − x } . {\displaystyle |x|=x\vee -x:=\sup\{x,-x\}{\text{.}}}