Deindividuation is when people lose their personal identity and their inhibitions about violence.
Zimbardo believes that being in a large group makes people feel anonymous and this lowers personal consequences for their actions. Being anonymous reduces inner restraints and people lose their own sense of personal self-awareness. This can increase behaviours like aggression that are usually inhibited when in an individuated state
Zimbardo distinguishes between individuated (rational behaviour which conforms to social norms) and deindividuated behaviour (impulsive based on more primitive urges).
Deindivduation gives people a cloak of anonymity that reduce personal consequences for their actions. Factors that encourage deindividuation are being part of a large group or altered consciousness due to drink or drugs.
Deindividuation is increased by many factors such as being in a large crowd, wearing uniform, being in darkness, and when wearing a mask, as all of these factors reduce our private sense of self-awareness.