# A Concept Which Has Been 'Specialized' In the Course of History

There are so many concepts which have been generalized during history of mathematics - the concept of "number" may be the best examples.

On the other hand, a concept may have been specialized ; the concept has referred to a wider range of examples in the mathematics of the past than present.

Do you know any such concept?

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It used to be not unheard of to consider "primes" to include $1$. –  Henning Makholm Mar 25 at 16:02
Roughly around the 1910s and 1920s the term function became specialized to what used to be known as a single-valued function. –  Dave L. Renfro Mar 25 at 16:28
@DaveL.Renfro Would you give your reference? –  Behzad Mar 25 at 16:50
The word set now has a technical meaning, out of necessity as shown by Russell. –  Zhen Lin Mar 25 at 17:29
@Behzad: I don't have a specific reference. Rather, this is something I've observed over many years of looking at thousands of journal articles and books from the early 1800s until the present. If it's very important to have a specific reference (for something you're working on?), I could probably find some references in a couple of days in some of the survey papers I have on the evolution of the function idea. Oh, I just noticed that I've previously posted a list of some of these papers. –  Dave L. Renfro Mar 25 at 18:16