For questions related to fuzzy set theory
In fuzzy set theory, elements have varying degrees of membership in sets.
Formally, a fuzzy set is a pair $(S,m)$ where $S$ is a set and $m\colon S\to[0.1]$. $m$ is a membership function. If $m(x)=0$, the element $x$ is not included in the set. If $m(x)=1$, then $x$ is fully included in the set.