Why the number 1 is not considered a prime number though it fits the definition?

The number 1, in fact, does not fit the definition of a prime number. A positive integer is called a prime number only if there are exactly two divisors of that number. Since 1 has exactly one divisor (which is 1 itself), it does not fit this definition. Another equivalent definition of a prime number is this: a prime number’s only positive divisor should be less than 1 and itself. Again, 1 does not fit this definition either – there are no positive divisors of 1 which are less than 1.