Ceteris Paribus:

The Latin phrase meaining "all other things being equal". The ceteris paribus assumption is more commonly used in economic theory to isolate the effect of a change in one variable or influencing factor. This implies that "all other variables or detemining factors are held constant". Economic theories are simplified and their validity enhanced with explicit or implicit use of ceteris paribus.

Rational Behaviour:

As used in economics, behaviour in which economic agents i.e. individuals, firms and govenment do the best they can under given circumstances. For example, the assumption of consumer rationality implies that the average comsumer in his purchasing decision will always preper more to less, or the basket of goods that will give him the maximum utility given his money income and the unit prices of the goods. The assumption of rationality permits us to explain and predict how people will act under specific conditions.