A person you work with. It is warm and informal — the -mate ending makes it feel friendly, like a person you would happily have lunch with, not just a name on the team list. Mostly British and Australian; Americans rarely say it and would say coworker or colleague. The three words differ in feel. Colleague is the formal one and hints at professional or office work — lawyers and teachers have colleagues. Coworker is neutral and works everywhere, and is the safe choice for Americans. Workmate is the friendly, spoken one, and is used most about hands-on or shift jobs: factory, warehouse, shop, kitchen. "My workmates threw me a party" sounds natural; "my workmates published a paper" sounds odd. It says nothing about rank — your workmate can be older or younger than you. In Taiwan, note that you will hear coworker far more in business English classes and emails, so treat workmate as something to understand rather than something you must use.

同事、工作夥伴（口語、親切的講法）。台灣職場英文更常用 colleague 或 coworker，workmate 聽懂即可。