Define singleton set.

Answer

### Step-by-Step Text Solution:

1. **Definition of a Set**: A set is a collection of distinct objects, considered as an object in its own right.

2. **Understanding Singleton Set**: A singleton set is a specific type of set that contains exactly one element.

3. **Example of a Singleton Set**: For instance, if we have a set A = {3}, this set A is a singleton set because it contains only one element, which is the number 3.

4. **Notation**: The number of elements in a singleton set is always 1. This can be denoted as |A| = 1, where A is the singleton set.

5. **Conclusion**: Therefore, a singleton set is defined as a set that has exactly one element.