Sorry, this is a basic question. I'm looking for a term which describes the following kind of trend.

Let's say a value in a time series peaks at a rate of 100 in the third year. The rates during the build-up phase are 50 in Year 2 and 25 in Year 1. In other words, during the build-up up to and including the first year of the peak, a predecessor is always half of its successor.

I'd be grateful for any help.