pseudodifference

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 http://www.hindawi.com/Getpdf.aspx?doi=10.1155/S0161171280000312this online article.