A scale of measurement where the position is marked using the logarithm of a value instead of the actual value.
It is useful because any equal multiplication has the same distance.
See below that the distance from 1 to 2 is the same as the distance from 2 to 4, or from 4 to 8
• the distance from 1 to 3 is the same as 3 to 9
• the distance from 1 to 10 is the same as 10 to 100, or 100 to 1000 etc.