TITLE: Why is $1$ not a prime number?
QUESTION [133 upvotes]: Why is $1$ not considered a prime number?
Or, why is the definition of prime numbers given for integers greater than $1$?

REPLY [2 votes]: The Question should be _"Why is the word $prime$ used only for a positive non-invertible indecomposable integer?"_The Answer is that Positive-NonInvertible-Indecomposable-Integer is too long, except in German. If you need a name for numbers that are either prime or $1$, you may invent one!