Just in case you were wondering I have decided to have "Catholic Definition Day". Here is the definition for today:
Heretic – a baptized person who rejects a dogma of the Catholic Church. Heretics are
automatically excommunicated from the Church (ipso facto) without any declaration for rejecting an authoritative teaching of the Faith.
Pope Leo XIII, Satis Cognitum (# 9), June 29, 1896:
“No one who merely disbelieves in all (these heresies) can for that reason regard himself
as a Catholic or call himself one. For there may be or arise some other heresies, which are
not set out in this work of ours, and, if any one holds to a single one of these he is not a
Catholic.” The Papal Encyclicals, by Claudia Carlen, Raleigh: The Pierian Press, 1990,Vol. 2 (1878-1903), Vol. 2 (1878-1903), p. 393.
Pope St. Pius X, Editae Saepe (# 43), May 26, 1910: “It is a certain, well-established fact that
no other crime so seriously offends God and provokes His greatest wrath as the vice of
heresy.” The Papal Encyclicals, Vol. 3 (1903-1939), p. 125.
Now heresy is not the same as "apostasy". The apostate abandons fully the faith of Christ. The heretic still maintains his faith in some fashion. So someone who rejects Christ and becomes a Mohammedan for instance would be an apostate. Someone who rejects some dogma of the Church but remains "Catholic" is a heretic.
Hope this helps!