Sports related term.
It is a team, which doesn't necessarily consisted of the best players, but the best team players with enough talent that the team itself is a juggernaut against opponents.
A loss would be in the most rarest in circumstances, and would be among the best of all time in sports.
1992 Olympic US Men's Basketball Team is a popular, and most known Dream Team.
The 1980 Olympic US Men's Hockey Team could be considered a Dream Team, considered that all players consisted of hand picked college players and went up against a USSR team which could be considered the best national hockey team ever.