Kantian refers to the philosophy of the German thinker Immanuel Kant (1724–1804). To say that something is Kantian — perhaps an ethical choice that you made — is to say that it aligns with the writings and beliefs of this important philosopher.
What does Kantian mean, from a philosophical perspective?
Kant's contributions to philosophy marked a great shift in how people thought of the world and how it worked. Two such contributions are the ideas of the autonomous will and the Categorical Imperative.
In his A Critique of Pure Reason, Kant tried to bridge the gap between the emipiricists, who believed that all true knowledge comes from experiences, and the rationalists, who believed that true knowledge comes from rational thought, regardless of experiences. In so doing, he stated that all rational wills — that is, all minds that were able to make choices based on reason — were autonomous, or capable of making their own choices. If all rational people are equally capable of making choices and deciding Right and Wrong, it therefore follows that all rational people are of equal value — king, commoner, and slave alike.
In Grounding for the Metaphysics of Morals, Kant sketched out his idea of the Categorical Imperative, a guideline by which any rational mind can decide between Right and Wrong. The Categorical Imperative states that the right moral choice is the one that could become a universal law without creating a logical fallacy.
Put more simply, when you're faced with a moral choice and trying to decide which course of action to take, consider each action and ask yourself, "What if everybody did it this way?" This is the idea of universalizability: It can't be Right for you if it wouldn't be Right for everyone else in a similar situation.
Kant's writings had an immediate and lasting effect on Western philosophy, including the beliefs of the Founding Fathers of the United States. It's easy to see the Kantian philosophy underlying this famous sentence from the Declaration of Independence: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness." The ideas of equality, liberty, and the pursuit of happiness all stem from the Kantian philosophy that a rational mind, given any common situation, is capable of making a correct ethical choice and should be allowed to do so.