What is a set? Well, simply put, it's a collection.
First we specify a common property among "things" (we define this word later) and then we gather up all the "things" that have this common property.
?https://www.mathsisfun.com/sets/sets-introduction.html
?what does property mean in this context?
?I don't get the proper meaning of characteristics and properties in science and maths.
com/en/definition/property An attribute, quality, or characteristic of something.
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Sense 2 at https://www.lexico.com/en/definition/property
An attribute, quality, or characteristic of something.
The properties of "1" are (for example)
The properties of "2" are not the same as "1". However they share two properties: #1 and #3.