GeezerVison is showing mathworld.wolfram.com — Set -- from the MathWorld Classroom

Set

In mathematics, a set is a finite or infinite collection of objects in which order has no significance and multiplicity is generally also ignored.

Set is a middle school-level concept.

Closed Set:	A closed set is a subset of a topological space that contains all of its limit points. A closed interval is an example of a closed set.
Open Set:	An open set is a set for which every point in the set has a neighborhood lying in the set. An open set is the complement of a closed set and. An open interval is an example of an open set.
Subset:	A subset is a portion of a set. Subsets can also consist of the entire set or be empty.