Analysis of information sources in references of the Wikipedia article "Equality (mathematics)" in English language version.
A set is a collection of definite distinct objects of our perception or of our thought, which are called elements of the set.
A relation between two quantities or other mathematical expressions stating that the two are the same; (also) an expression of such a relation by means of symbols, an equation.
[...] the most common axiom system was and is called the Zermelo-Fraenkel system.
[...] the most common axiom system was and is called the Zermelo-Fraenkel system.