Notation and Terminology
The relation "is an element of", also called set membership, is denoted by ∈. Writing
means that "x is an element of A". Equivalent expressions are "x is a member of A", "x belongs to A", " is in A" and "x lies in A". The expressions "A includes x" and "A contains x" are also used to mean set membership, however some authors use them to mean instead "x is a subset of A". Logician George Boolos strongly urged that "contains" be used for membership only and "includes" for the subset relation only.
The LaTeX command for this symbol is "\in".
The negation of set membership is denoted by ∉.
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