It’s common to think of skepticism simply as doubt—and as far as one-word definitions go, that’s fine—but it’s not quite as simple as that. Skepticism is not simply a position on a subject: it’s a method. It’s a method of critically examining and analysing claims, to sort those that are probably true from those that are probably not.
Modern skepticism is closely linked with science, and for that reason is commonly called “scientific skepticism”. Celebrated astronomer and skeptic Carl Sagan summarized it thus: “Extraordinary claims require extraordinary evidence.”