As the other members have said, "mate" is a casual term for "friend." In the US it's not common except in expressions like "classmate" (同級生), "roommate" (ルームメイト), and sometimes to refer to a spouse (結婚相手 or 伴侶). In the US, people are more likely to use words like "friend," or sometimes "buddy."

thanks:)

It’s casual. You would use it with your friends. I think it’s more common in England than in other parts of the UK. I started using it when I moved from Scotland to London. It’s very useful - you can use it when you can’t remember somebody’s name! “Hey mate, how have you been?”

友達、仲間くらいの意味ですよ。アメリカの英語(男の子)だったら、brother(親しみを込めた兄弟 本当の兄弟てはない) のくらいの意味じゃないでしょうか。オーストラリアでもmateは使われています。礼儀正しい感じの使い方ではなく、フランクな言い方です。