Describes a person, attitude, or decision based on reason rather than emotion. Often used in contrast with 感情的 (emotional).
He always makes rational decisions.
Let's discuss this rationally, without getting emotional.
理性的 describes a person's calm, reason-driven character or state of mind, whereas 合理的 typically describes decisions, systems, or explanations.
理性的 describes acting on reason and logic, directly opposed to 感情的 behavior driven by feelings.
Compound of 理性 (reason) + 的 (-like, -ical). 理性 is from Chinese 理性 (lǐxìng), and 的 is a Sino-Japanese suffix forming adjectival nouns.