i believe the answer is "scale factor." you usually find it by comparing the corresponding sides from two figures and creating a ratio of them; for example, imagine two squares. square A has sides that have the length of 8 inches. square B has sides that have a length of 4 inches. all sides are the same length with squares.
square A, length of left side = 8 in
square B, length of left side = 4 in
the ratio for this could be 8/4 (simplified to whole number 2) -- you would pick two corresponding sides (the left sides of each square) and think "when the left side on square A is 8 in, the left side on square B is 4 in. if i multiply 4 in by 2, i get the larger square, so square B is smaller by a factor of 2."