Scale Factor is a ratio used as a multiplier/dividend in scaling (make an object bigger or smaller).
It’s the ratio of any two corresponding lengths in two similar figures.
These two figures on the left are similar figures because they maintain the same ratios. 16 = 32, 25 = 50, 22 = 44. Each side of the larger triangle is double the size of the smaller triangle, therefore the scale factor is 2.