Humanism is a progressive lifestance that, without supernaturalism, affirms our ability and responsibility to lead meaningful, ethical lives capable of adding to the greater good of humanity.
Humanism is a rational philosophy informed by science, inspired by art, and motivated by compassion.
Affirming the dignity of each human being, it supports the maximization of individual liberty and opportunity consonant with social and planetary responsibility.
It advocates the extension of participatory democracy and the expansion of the open society, standing for human rights and social justice.
Free of supernaturalism, it recognizes human beings as a part of nature and holds that values–be they religious, ethical, social, or political–have their source in human experience and culture.
Humanism thus derives the goals of life from human need and interest rather than from theological or ideological abstractions, and asserts that humanity must take responsibility for its own destiny.
— The American Humanist Association
For a more detailed background about the history of Humanism and the various philosophies that make up Humanism check out the relevant wikipedia article on Humanism. It’s one of the most thorough explanations of all that Humanism was and is.