The story
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: Of a relation ~ on a set S: such that every element that is related to some element is also related to itself, i.e. ∀x,y∈S: x~y ⇒ x~x ∧ y~y. Full definition →
Etymology from Wiktionary, CC BY-SA 4.0.