(Latin: definire, to enclose within limits)
A solemn and irrevocable decision emanating from the supreme teaching authority of the Church (the pope by his own authority or as presiding over an Æcumenical Council), concerning a matter of faith and morals, and made binding on all the faithful.
The object of a definition may be either a doctrine revealed by God, and contained in the deposit of faith, which the Church has the duty to guard and propose authoritatively; or it may be a truth not so revealed but intimately bound up with revealed doctrine.
Such definitions demand the unconditional adherence of every member of the Church.
New Catholic Dictionary