Cēterīs paribus is derived from the Latin, and is literally translated as "with other things [being] the same." This is usually expressed in English as "all other things being equal."
A ceteris paribus assumption concerns causal or logical connections, and acknowledges – so as to rule out – the possibility of extraneous, confounding factors that could override the causal relationship between the antecedent and the consequent.
Ceteris paribus assumptions are often fundamental to the predictive purpose of scientific inquiry, where formulation of scientific laws requires the elimination of confounding factors that could interfere with examining a specific causal relationship.
Experimentally, the ceteris paribus assumption is realized when a scientist controls for all of the independent variables other than the one under study. This isolates the impact on the dependent variable under investigation to the operation of a single independent variable. By holding other relevant factors constant, a scientist hopes to focus on the unique effects of a specific factor in a complex causal situation. (Obviously, any as yet unrecognized independent variables could continue to confound results.)
For the same reasons, simplifying assumptions are also relevant to the descriptive purpose of modeling theories within analytical frameworks that describe fundamental concepts within fields such as economics, physics, and behavioral psychology.