An element $z$ of lattice $L$ is pseudodifference of $y$ and $x$ ($y\setminus x$ if $z$ is the least element such that $y\le x\cup z$

Pseudodifference is denoted either as $\setminus$ or as $-$ Sometimes pseudodifference is denoted as $\setminus^*$

The definition is borrowed from this online article.