The rationale for something is the basic or underlying reason or explanation for it. This noun (pronounced "rash-uh-NAL") is usually used in the singular: What was the rationale behind his decision to quit?
The related adjective rational means "based on facts or reason" or "having the ability to think clearly." An example of a rational rationale? You add extra baking soda to pancake batter under the rationale that if you want them to be fluffier, you need more rising agent. Rationale is from Latin, from rationalis "relating to reason," from ratio "calculation, reason," from rērī "to consider, think."