From a Greek word signifying (1) a choice, (2) the opinion chosen, and (3) the sect holding the opinion. In the Acts of the Apostles (5:17; 15:5; 24:5,14; 26:5) it denotes a sect, without reference to its character. Elsewhere, however, in the New Testament it has a different meaning attached to it. Paul ranks "heresies" with crimes and seditions (Galatians 5:20). This word also denotes divisions or schisms in the church (1 Corinthians 11:19). In Titus 3:10 a "heretical person" is one who follows his own self-willed "questions," and who is to be avoided. Heresies thus came to signify self-chosen doctrines not emanating from God (2 Peter 2:1).
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Heresy'". "Easton's Bible Dictionary".