Greatest Common Factor
The highest number that divides exactly into two or more numbers.
If you find all the factors of two or more numbers, and some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.
Abbreviated "GCF". Also called "Highest Common Factor"
Example: the GCF of 12 and 30 is 6, because 1, 2, 3 and 6 are factors of both 12 and 30, and 6 is the greatest.