The phrase "defining mark" refers to a unique quality or characteristic that distinguishes someone or something from others. It is something that sets them apart and helps identify who they are or what they represent. Full definition
The only defining mark on the car sits on the trunk lid where a small badge with the letter H sits just over the right tail light. (autobytel.com)
The ritual observance of dietary rules — fasting and abstinence from meat in Lent, and abstinence from meat and meat products every Friday, as well as the eucharistic fast from midnight before the reception of Communion — were as much defining marks of Catholicism before the council as abstention from pork is a defining characteristic of Judaism. (firstthings.com)
New Zealanders using have found meeting Can Christians Use Birth Control The separation of sex from procreation may be one of the most defining marks of our age — and one of the most ominous. (myintimcity.com)