A liberal was an individual who practiced liberalism.
In 1953, Douglas Pabst dismissed the works of W.E.B. DuBois, Zora Neale Hurston, Langston Hughes, Ralph Ellison, and Richard Wright as "literature for liberals and intellectuals", saying the average reader wouldn't spend their money on Negro-written stories. (DS9: "Far Beyond the Stars")
In 2366, while studying various parenting methods after creating a daughter for himself, Data noted the certain liberal approaches allowed one's child to have "enormous freedom". (TNG: "The Offspring")