Abnegation is one of the five factions in the world of Divergent. It is the faction dedicated to selflessness and forgetting oneself for the sake of others.
Their substance in the Choosing Ceremony are grey stones.
The Abnegation typically wear grey. They are the faction that believes in helping others and being selfless. Most Abnegation children brought up never looking in the mirror. They wear clothes and hair styles that don't stand out because they believe that would be self indulgent and against the faction norm.
To "abnegate" is to deny or renounce oneself.