From ACT Wiki
1. Futures trading.
In futures or options trading, an unusual market condition in which longer-term contracts carry a lower price than near-term contracts.
(The usual relationship - known as contango - is that longer-term contracts carry a higher price than near-term contracts.)
2. Foreign exchange - spot price.
The extent to which a spot price of a foreign currency plus carrying cost exceeds the forward price.
3. Market conditions - price relationships.
More generally, any market conditions under which related market prices do not have their usual relationships to one other, potentially creating an arbitrage opportunity.
(Also sometimes known as a 'back price'.)