What is Basis Trading?

Legal Definition
Basis trading is a financial trading strategy which consists of the purchase of a particular financial instrument or commodity and the sale of its related derivative (for example the purchase of a particular bond and the sale of a related futures contract).

Basis trading is done when the investor feels that the two instruments are mispriced relative to one other and that the mispricing will correct itself so that the gain on one side of the trade will more than cancel out the loss on the other side of the trade. In the case of such a trade taking place on a security and its related futures contract, the trade will be profitable if the purchase price plus the net cost of carry is less than the futures price.
-- Wikipedia
Legal Definition
Tranding where an investor buys a security and sells a similar one. A gain is made from mispricing.