Page "Countable set" Paragraph 6

A set is a collection of elements, and may be described in many ways.

One way is simply to list all of its elements ; for example, the set consisting of the integers 3, 4, and 5 may be denoted.

This is only effective for small sets, however ; for larger sets, this would be time-consuming and error-prone.

Instead of listing every single element, sometimes an ellipsis ("...") is used, if the writer believes that the reader can easily guess what is missing ; for example, presumably denotes the set of integers from 1 to 100.

Even in this case, however, it is still possible to list all the elements, because the set is finite ; it has a specific number of elements.