A tuba designed for marching that resembles a concert tuba in shape but is carried on the left shoulder. They usually weigh between 35 and 55 pounds.
The visual advantage for such a tuba on the field is that it can be held in carry (held vertically in front of the person) and in various other positions.
They are called Contras for short.
See the pic above.
Contras are the most badass instruments ever.