Page "Zermelo–Fraenkel set theory" Paragraph 2

Formally, ZFC is a one-sorted theory in first-order logic.

The signature has equality and a single primitive binary relation, set membership, which is usually denoted ∈.

The formula a ∈ b means that the set a is a member of the set b ( which is also read, " a is an element of b " or " a is in b ").