Reason: (1) The ability to understand and explain cogently, based on evidence and according to logical principles; (2) the ability to treat others fairly and decently, unless one is harmed by them.
1. This is a fundamental human capacity, and based on the capacity to represent things symbolically. A cogent explanation is one that is based on true or probable premisses and deductively entails what it explains. Science is based on reason, and the test that something is a real science is that it has produced a real technology that works independent of belief in or understanding of the science that produced it. By contrast, faith - whether religious or political - is not based on reason but on wishful thinking, and has no technology at all, except violence and whatever can be based on superstition and groupthinking.
There are three basic kinds of reasoning, where reasoning involves argumentation of any kind using assumptions and inferences of conclusions:
1. Deductions: To find conclusions that follow from given assumptions
2. Abductions: To find assumptions from which given given conclusions follows
3. Inductions: To confirm or infirm assumptions by showing their conclusions do (not) conform to the observable facts.
Normally in reasoning all three kinds are involved: We explain supposed facts by abductions; we check the abduced assumptions by deductions of the facts they were to explain; and we test the assumptions arrived by deducing consequences and then revising by inductions the probabilities of the assumptions by probabilistic reasoning when these consequences are verified or falsified.
2. The term "reason" is used in another sense, that is more related to morals and ethics than to science. In this sense, one is reasonable if one treats others fairly, does not harm them unless attacked, does not deceive them without provocation, and in general behaves towards them according to some schema of values that chart what it is to be virtuous.