Delta is a ratio that indicates the sensitivity of an option's premium to variations in the share price of the underlying asset. It indicates how much the option's premium varies when the price of the underlying asset increases by 1 euro. A Delta with a high absolute value implies that the option's premium has a higher sensitivity to changes in the quoted price of the underlying asset. An option's Delta represents the premium's speed of change.
It can also be considered as the probability that any option is in-the-money (has intrinsic value) by its expiration date.