Definition: A set having only one element is called a singleton.
These sets are singletons: (i) A={8}, (ii) B={x|x is an even prime number}→B={2}

Singleton (noun)
A set, consisting of a single element, is called a singleton.

Example 1. If a set C has only one element, we call it a singleton. C={a}. Thus, {a} is a singleton.

Ex2. The sets {0}, {5}, {-7} are singletons.

Ex3. D={x∶x+8=0,x∈ℤ} is a singleton, because this set contains only one integer, namely -8.

Ex4. Find, which of the following sets are singletons:
(i) The set of points of intersection of two non-parallel straight lines on the same plane.
(ii) E={x∶7x-3=11}
(iii) F={y∶2y+1<3 and y∈𝕎}
Solution:
(i) The set of points of intersection of two non-parallel straight lines on the same plane is a singleton.
(ii) E={x∶7x-3=11}

Hence, the given set E has only one element, so it is a singleton.

(iii) F={y∶2y+1<3 and y∈𝕎}

Hence, it is a singleton.