If you play a game or a sport, anyone on your team is your teammate. During a game of charades, you might silently act out the title of a movie while your teammates try to guess it.
All the basketball players on the Los Angeles Sparks are teammates. Doubles tennis players who play together against another pair are also teammates. You can use the word to describe someone who collaborates with you, too, like your teammates at work or a teammate on a school project. The word teammate can be traced back to the Old English team, "family, brood, or company."