Definition of average rate:

The average rate at which quantity A changes with respect to quantity B on an interval is

average rate = (change in A) / (change in B).

Technicality/abstraction that you should probably ignore right now:  The word 'interval' is for the moment not defined, but in general an interval can be thought of as (a closed interval) being bounded by an 'initial event' and a 'final event'.  The word 'between' implies that there is some way of ordering events.