Definition
Informal slang for a male friend or peer, or a casual way to address someone; can also denote a stereotypically masculine, frat/party-oriented “bro culture” persona.
Examples
"Bro, can you pass me the charger?"
"I'm grabbing tacos with my bro after the game."
"He's such a bro—backwards cap, protein shakes, and plans for the next frat party."