Sounds like "exponential decay" is the answer your teacher is looking for
Expressions of the form y = a*b^x are considered decay equations or exponential decay equations if 0 < b < 1. So b can be between 0 and 1, but not equal to either endpoint.
Example: y = 3*0.5^x means we start off with 3 as the initial value, and then cut it in half repeatedly as x increases by 1 (eg: x = 0, x = 1, etc). This graph goes downhill as you read it from left to right.