From Why start at x, y, z

The factors of 21 are usually 1, 3, 7, and 21, but sometimes factors is used to describe the number 1, 3, 7 (excluding the number itself).

The proper factors of 21 might be 1, 3, and 7[1]; or 3 and 7[2].

"Factor" and "divisor" are often used interchangably, but are given distinct meanings by others: Wolfram MathWorld states that proper factors exclude 1 and the number itself, while proper divisors include 1 and exclude the number itself.

There is also inconsistency over whether to include negative numbers as factors or divisors.