Greek: αιρεσις [hairesis] (Noun Feminine). Choise, preference [inclination] by a doctrine or opinion, or philosophical, theological or political school. By extension, "party" or "faction". (Of the original meaning ["election", "selection"], happened to mean "that which is chosen", "opinion"; and later it got to mean a group of people who maintained a particular opinion, a sect, a side). From this word it derives "heresy", word that originally didn't have the pejorative sense that the ecclesiastical authorities gave him later.

αιρεσις [hairesis] appears 9 times in the NT:
- heresy 4
- sect 5

(1) Act 5: 17
Then the high priest rose up, and all they that were with him, (which is the sect of the Sadducees,) and were filled with indignation, - αιρεσις [hairesis] Nom. sing.

(2) Act 15: 5
But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses. - των απο της αιρεσεως [tôn apo tês haireseôs] Gen. sing.

(3) Act 24: 5
For we have found this man a pestilent fellow, and a mover of sedition among all the Jews throughout the world, and a ringleader of the sect of the Nazarenes: - της αιρεσεως [tês haireseôs] Gen. sing.

(4) Act 24: 14
But this I confess unto thee, that after the way which they call heresy, so worship I the God of my fathers, believing all things which are written in the law and in the prophets: - αιρεσιν [hairesin] Accus. sing.

(5) Act 26: 5
Which knew me from the beginning, if they would testify, that after the most straitest sect of our religion I lived a Pharisee. - την αιρεσιν [tên hairesin] Accus. sing.

(6) Act 28: 22
But we desire to hear of thee what thou thinkest: for as concerning this sect, we know that every where it is spoken against. - της αιρεσεως ταυτης [tês haireseôs tautês] Gen. sing.

(7) 1 Corinthians 11: 19
For there must be also heresies among you, that they which are approved may be made manifest among you. - αιρεσεις [haireseis] Nom. pl.

(8) Galatians 5: 20
Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, - αιρεσεις [haireseis] Nom. pl.

(9) 2 Peter 2: 1
But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction - αιρεσεις [haireseis] Acus. pl.