ratio scale

an interval scale with a true zero.

A ratio scale has two distinct properties: distances on the scale are equivalent, and the scale has a true zero. In terms of distances on the scale, take time, which is a ratio scale. The difference between 1 and 2 second is the same as the difference between 21 and 22 seconds: exactly one second. If that seems obvious, think about marathon racers crossing the finish line. The difference between 1 and 2 might be huge, but the difference between 21 and 22 might not: they are not equivalent.

Absolute zero: it makes sense to talk about something taking "twice as long" as something else. A time of 4 hours is twices as big as a time of 2 hours. This is because zero has a real meaning for an interval scale. In contrast, look at a scale that is not ratio, like temperature. 60 degrees is not "twice as hot" as 30 degrees. The zero in temperature has no special significance.