Delta
An estimate of how much an option's price changes for a $1 move in the underlying. Long calls have positive delta and long puts have negative delta.
Delta runs from 0 to 1 for calls and from 0 to -1 for puts. An at-the-money option has a delta near 0.50 in absolute terms, deep in-the-money options approach 1, and far out-of-the-money options approach 0.
Delta changes as the underlying moves, as time passes and as implied volatility changes, so it is a local estimate. Traders also use delta as a rough probability that an option finishes in the money, and multiply it by 100 to express a position in share-equivalent terms.
Illustrative example (hypothetical numbers, not a real trade): a call with a delta of 0.40 trades at $2.00. If the underlying rises $1 and nothing else changes, the call moves to about $2.40. Ten of these contracts carry a delta of 10 × 0.40 × 100 = 400 shares.