Christopher Avery, who has written a book about teams called “Teamwork is an Individual Skill”, has a good reference page on his web site about teams. From his FAQ, here is his definition of “team”:

A team is a group of people whose personal outcomes are obviously linked to a collective outcome — such as a successful project — and who work together to maximize collective and individual outcomes. “Team” also refers to the quality of group relationships that allows ordinary individuals to achieve extraordinary results together — such as a project that surpasses its goals.