When talking about a limiting factor we assume all other variables are infinite. It is the limiting factor because there is the least of it. You described having increasing substrate, which leaves enzyme concentration as the limiting factor.
Substrate becomes the limiting factor when it is what is slowing the reaction down, if there is only a few bits of substrate in a cell, there is less chance of them bumping into an enzyme, so reactions will occur slower, it is the limiting factor. If you then increase substrate concentrations there will be more substrate floating around, therefore they will bump into enzymes more frequently, thus speeding up the reaction.
As for key terms, I just made sure to name processes. Normally the names of processes are what you need to include. For example, don’t say a protein was packaged into a vesicle and travelled to the membrane where it was released, just say a protein left the cell via exocytosis. (Obviously this doesn’t apply if the question is ‘describe how a protein leaves a cell’)