What is Crack Spread?

Legal Definition
Crack spread is a term used on the oil industry and futures trading for the differential between the price of crude oil and petroleum products extracted from it. The spread approximates the profit margin that an oil refinery can expect to make by "cracking" the long-chain hydrocarbons of crude oil into useful shorter-chain petroleum products.

In the futures markets, the "crack spread" is a specific spread trade involving simultaneously buying and selling contracts in crude oil and one or more derivative products, typically gasoline and heating oil. Oil refineries may trade a crack spread to hedge the price risk of their operations, while speculators attempt to profit from changes in the oil/gasoline price differential.
-- Wikipedia
Legal Definition
A SPREAD in the energy market reflecting the price differential between crude oil and a refined product, generally gasoline or heating oil; the spread can be traded through a single FUTURE or OPTION contract on certain EXCHANGES. A hedger or speculator can buy the crack spread (e.g., purchase crude and sell heating oil or gas) to take advantage of positive margins in refining, and sell the spread (e.g., sell crude and purchase heating oil or gas) to profit from negative margins. See also SPARK SPREAD.