An aggregate, totality, collection of any objects whatever, called its elements, which have a common characteristic property. "A set is many, conceivable to us as one" (G. Cantor). This is not in a true sense a logical definition of the notion of a set, rather it is just an explanation (because defining the notion means finding a generic idea to which the given idea belongs as a species; but a set is, unfortunately, itself a broad notion in mathematics and logic). In this connection it is possible either to give a list of the elements of the set, an enumeration of it, or to give a rule for determining whether or not a given object belongs to the set considered, a description of it (moreover, the first is really acceptable only when the question concerns finite sets).
For a meaningful development of "naive" set theory such an explanation is quite sufficient, because for the mathematical theory it is only essential to define the relations between the elements of a set (or between the sets themselves), and not their nature. To describe sets which may be elements of another set, to avoid the so-called antinomies (cf. Antinomy) one introduces, for example, the terminology "class" . And then, speaking more formally, set theory deals with objects called classes (cf. Class), for which there is defined a relation of membership, and a set itself is defined as a class which is an element of some class.
Recently the unifying role of category theory (and, in particular, the notion of a universal set) has become more clear. The construction of a category is based on axiomatic set theory and allows one to consider, for example, such "large" collections as the category of all sets, groups, topological spaces, etc.
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Set. M.I. Voitsekhovskii (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Set&oldid=11955