“A set[1] is closed under an operation if performance of that operation on members of the set always produces a member of that set. For example, the positive integers are closed under addition, but not under subtraction: 
“Similarly, a set is said to be closed under a collection of operations if it is closed under each of the operations individually” (“Closure (mathematics),” Wikipedia, retrieved 10/9/2020, bold added or taken away for emphasis).
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[1] See “What is the Gist of a “Set” (Mathematics)?“
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Disclaimer:
I am not a professional in this field, nor do I claim to know all of the jargon that is typically used in this field. I am not summarizing my sources; I simply read from a variety of websites until I feel like I understand enough about a topic to move on to what I actually wanted to learn. If I am inaccurate in what I say or you know a better, simpler way to explain a concept, I would be happy to hear from you :).
