Believing that no gender is superior or inferior to the other simply for being that gender. Believing that women deserve to be treated equally to men, and have the same rights. Believing that all women have the choice and free will to do whatever they want, and make their own life decisions, and not be put down for it. Anything else isn't feminism and shouldn't be called feminism.
A movement/ philosophy developed in the late 19th century that has gone through several iterations and divided itself into several different subgroups since then, some more respectable than others.