I hate to date myself, but it seems that when I first learned algebra, a prime number was defined as "an integer that can only be divided evenly by itself and 1."

Now it appears that 1 has been excluded from the set of all prime numbers because a prime number is "an integer that can be obtained only by multiplying two different integers, itself and 1." Because 1 is not different from 1, 1 is thus not a prime number.

Does anyone know when 1 was excluded formally from the set of all prime numbers and what the reason was for doing it?