Georg Cantor, in 1894. Cantor was the first mathematician to talk about sets

Cantor's original definition of a set

A set is an idea from mathematics. A set has members (also called elements). A set is defined by its members, so any two sets with the same members are the same (e.g., if set \displaystyle{ \mathit{X} } and set \displaystyle{ \mathit{Y} } have the same members, then \displaystyle{ \mathit{X} = \mathit{Y} }).

Example of a set of polygons