Fill in the blank.
A number which has only two factors is called a ______.

Hint: In this question we have to find the type of a number which has only two factors. It means that the number can be written as the product of two numbers. The only exception is 1 because it can not be written as the product of two numbers.

Complete step-by-step answer:
Given:
If a number has only two factors then it means that the number is divisible by only those two numbers. We know that any number is completely divisible by itself, so one of the factors of the original number is the number itself. Also, we know that any number is completely divisible by 1, so the second factor of the original number is 1.
So, the number that is completely divided by itself as well as 1, and can be written as the product of two numbers then that number is known as the “Prime Number”, this is the definition of the prime number. A prime number can also be written by the product of two numbers.
The examples of the prime numbers are - 2, 3, 5, 7, 11 etc.

Therefore, A number which has only two factors is called a “Prime Number”.

Note: It should be noted that 1 is not a prime number because we can not write 1 as the product of two numbers and according to the definition of the prime number any prime number can be written as the product of two numbers. So according to this condition 1 does not satisfy the definition of the prime number, therefore 1 is not a prime number.