Fr. Hardon, S.J.'s definition of "humanism" distinguishes "pagan humanism" and "Christian humanism":
Name originally given to the intellectual, literary, and scientific movements of the fourteenth century through the early sixteenth. Their aim was to base every branch of learning on the culture of classical Greek and Roman antiquity. On its pagan side, it extolled the early non-Christian writers who stressed the full development of human nature, only vaguely interested in life after death. On its Christian side, believing humanists encouraged the free use of the treasures of antiquity without compromising the truths of the Gospel. Christian humanism began with Dante (1265-1321), while pagan humanism reached its peak in Petrarch (1304-74). Popes Pius II, Sixtus IV, and Leo X favored Christian humanism and did much to promote it. St. Thomas More (1478-1535) typified its best spirit in England. After the French Revolution the extreme humanistic spirit rebelled against Christian revelation and the Church.
"Pagan humanism," since it denies or neglects the supernatural order, is naturalism, which Pope Pius IX condemned in his Syllabus of Errors and Pope Leo XIII condemned in his encyclical Humanum Genus.