Delta

Delta /ˈdɛltə/ (n.)
For the technical term relating to hedging, see deltaone
A voguish way celery peddlers convey the idea of “difference”. In fairness, “delta” does mean, more or less, “difference”, but there’s a less mystifying way of getting across that idea: The word, “difference”.
Technical answer
The option delta of a derivative is the ratio between a change in the price of that derivative and the change in price of the underlying asset it is a derivative of.
Delta values range from 1.0 to 1.0.
 A delta of 1.0 gives an exact correlation with the performance of the underlying. A call option necessarily has positive delta: as the underlying asset increases in price, the call value also increases.
 A delta of 1.0 does the exact opposite of what the underlyer is doing. A put option necessarily has a negative delta. Well of course it does: you shorted the underlyer. As the underlying security increases in value, your put goes out of the money.
 A delta of 0 means the option and the underlyer are not correlated at all: their performance with respect to each other is random. A derivative with a delta of nil basically isn’t a derivative of that underlying.
Technically, the value of the option’s delta is the first derivative of the value of option with respect to the underlying security’s price.
See also
 Greeks  the home of all things Greek on this site. It’s our own little Athens.
 Alpha
 Beta
 Delta
 Nu  a trick for young players — and those with a degree in Classics.
 Omega — the end of days, and the right time for backtesting
 Vega — not really a Greek at all, but a maudlin singersongwriter
 Enhanced beta
 Leveraged alpha