In order to study how changing price affects consumer decisions, we must assume all other factors, such as income and the prices of other goods are constant. This assumption is best know as

The correct answer is Ceteris paribus

Ceteris Paribus, is a concept of the Economy, which translated means, "everything more constant".

In this way, Ceteris Paribus becomes a very important concept, as some studies that contain a large number of variables can be simplified.

In other words, you keep some variables constant, while you change others to identify their impact on the whole.