In Magic, a team is a group of players who share a common victory condition in a multiplayer game.
From the glossary of the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- A group of players who share a common victory condition in a multiplayer game. See rule 808, “Team vs. Team Variant,” rule 809, “Emperor Variant,” rule 810, “Two-Headed Giant Variant,” and rule 811, “Alternating Teams Variant.”
In Professional play, most Magic players also work with a team of some sort, to brew and playtest.