Liberalism in One Sentence
Liberalism in one sentence: "X is the morally right thing to say, therefore X is true."
EDIT: Of course, there's more to it than that. In some ways, that's inaccurate. In some cases, liberals are primarily concerned with "optics". Therefore, they aren't so concerned with whether something is actually morally right - they're only concerned with whether it looks morally right. (As Debord put it, the slogan of the spectacle is "That which appears is good, that which is good appears" - thus liberalism is inextricably tied up with spectacle.) "That's a bad look" is something that liberals say (the honest ones). But other liberals really have a politics that consists of trying to have good intentions and think good thoughts. But it doesn't really matter. The cynical liberal, who is only concerned with optics, and the idealistic liberal, who genuinely wants to be morally right, are effectively the same thing.
FURTHER EDIT: When I say "morally," I am referring specifically to bourgeois morality. Liberalism is the ideology of the ruling class, that is to say, the bourgeois class. (There are plenty of people who say, and have said, over the centuries, "X is the morally right thing to say, therefore X is true" - but from the perspective of different moral systems: Islamist theocracy, say, or Confucianism.) When I say "the ideology of the ruling class," I don't necessarily mean that everyone in the ruling class believes in, advocates for, understands, or lives by this ideology and its morality. Indeed, most of them do not. I simply mean that it is an ideology that represents the interests of this class. Liberalism is the working out, in increasingly fine detail, of this ideology - in common law, precedent, constitutional law, manners, traditions, customs, rhetoric, art, fashion, popular culture, and so on.
ONE MORE THING: A perfect example of liberalism, as I am describing it here, is the work of Immanuel Kant.
Other examples include Ayn Rand, the Austrian economic tradition, Keynesians, Utilitarians, postmodernists, and so on.