From the American Humanist Association Homepage:

Definitions of Humanism

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

Humanism is a democratic and ethical life stance which affirms that human beings have the right and responsibility to give meaning and shape to their own lives. It stands for the building of a more humane society through an ethics based on human and other natural values in a spirit of reason and free inquiry through human capabilities. It is not theistic, and it does not accept supernatural views of reality. -- The International Humanist and Ethical Union

Humanism is a philosophy, world view, or life stance based on naturalism--the conviction that the universe or nature is all that exists or is real. Humanism serves, for many humanists, some of the psychological and social functions of a religion, but without belief in deities, transcendental entities, miracles, life after death, and the supernatural. Humanists seek to understand the universe by using science and its methods of critical inquiry--logical reasoning, empirical evidence, and skeptical evaluation of conjectures and conclusions--to obtain reliable knowledge. Humanists affirm that humans have the freedom and obligation to give meaning, value, and purpose to their lives by their own independent thought, free inquiry, and responsible, creative activity. Humanists stand for the building of a more humane, just, compassionate, and democratic society using a pragmatic ethics based on human reason, experience, and reliable knowledge--an ethics that judges the consequences of human actions by the well-being of all life on Earth. -- The Virtual Community of Humanists