Definition:Singleton

A singleton is a set that contains exactly one element.

Some authors use the term unit set.

The singleton containing only the element $$a$$ can be written $$\left\{{a}\right\}$$.

When this happens, we must be careful to distinguish between the element itself, that is, $$a$$, and the set containing it, that is, $$\left\{{a}\right\}$$.