Hairesis (G139) originally meant 'a choice' or 'a party one chooses to join' (from haireō: to choose). In Acts it refers neutrally to the sects of Sadducees and Pharisees. But in Paul and Peter it takes on the negative sense of divisive factions that cause schism in the church — choosing one's own opinion over apostolic truth.
The evolution of hairesis to our word 'heresy' is significant. A heretic is literally someone who has made a personal choice — selecting beliefs contrary to the received apostolic deposit. The NT warns that haireseis will arise from within the church (2 Peter 2:1; 1 Corinthians 11:19). The antidote is not mere tolerance but holding firmly to sound doctrine handed down from the apostles.