"Secularism is a code of duty pertaining to this life, founded on considerations purely human, and intended mainly for those who find theology indefinite or inadequate, unreliable or unbelievable. Its essential principles are three: (1) The improvement of this life by material means. (2) That science is the available Providence of man. (3) That it is good to do good. Whether there be other good or not, the good of the present life is good, and it is good to seek that good." - George Holyoake, English Secularism (1846)
Holyoake is credited with coining/popularizing the modern usage of the term secularism.