Delta
Delta is the rate of change in the price of an option compared to the rate of change in the price of the underlying asset. For example, a delta of 0.50 on a call option means that the option price increases by $0.50 for each increase of $1.00 in the underlying asset. Deltas on call options range from 0.00 to +1.00, while deltas on put options range from 0.00 to -1.00.
See also: Greeks