Contango is a term used in the futures market to describe an upward sloping forward curve (as in the normal yield curve). Such a forward curve is said to be "in contango" (or sometimes "contangoed").
Formally, it is the situation where, and the amount by which, the price of a commodity for future delivery is higher than the spot price, or a far future delivery price higher than a nearer future delivery. This is a normal situation for equity markets.
The opposite market condition to contango is known as backwardation.