Infinity Symbol (∞)
Mathematical symbol representing an unbounded quantity or the concept of endlessness.
Mathematical/Western · 1655 – present
Element Of (∈)
Set theory symbol indicating that an element belongs to a set.
Description
The element-of symbol (∈) is used in set theory to indicate membership: the expression 'a ∈ A' means 'a is an element of set A'. The symbol was introduced by Giuseppe Peano in his 1889 work 'Arithmetices principia, nova methodo exposita', derived from the Greek letter epsilon (ε) for the word 'est' (is). The negated form (∉) means 'is not an element of'. Related symbols include ⊂ (subset of), ⊃ (superset of), and ∪ (union).
Origin & Cultural Context
Introduced by Giuseppe Peano in 'Arithmetices principia, nova methodo exposita' (1889).