1. to associate or interact in a friendly way with someone, especially someone from a different group or background
Usage: British spelling; American spelling is 'fraternize'; often used with 'with'
2. to have friendly relations with an enemy or opposing group, especially in a military context
Usage: often implies disapproval or violation of rules

Examples

• The soldiers were forbidden to fraternise with the local population.
• She enjoyed the chance to fraternise with colleagues from different departments at the conference.
• During the ceasefire, some troops began to fraternise with the enemy soldiers.
• The club encourages members to fraternise and build lasting friendships.
• He was reluctant to fraternise with the rival team after the match.
• Students from different schools fraternised at the summer camp.