A rationale is the underlying reason or logical explanation for why someone does something or believes something. When your teacher asks you to show your rationale for an answer in math class, she wants to see the thinking and steps that led you to that solution.

Your rationale is essentially your reasoning spelled out. If you want to convince your parents to let you get a dog, you'd better have a solid rationale: maybe you'll explain how you're old enough to handle the responsibility, how a dog would get you outside and exercising more, and how you've researched what breed would work best for your family.

Scientists always provide a rationale for their experiments, explaining why their research matters and what they hope to discover. A city council provides a rationale when deciding where to build a new park. A coach explains the rationale behind a new training program.

The stronger your rationale, the more convincing your argument becomes. A weak rationale might be “because I want to” or “everyone else is doing it.” A strong rationale connects your decision to solid reasons that others can understand and evaluate, even if they don't ultimately agree with you.