How much an option's price moves per $1 move in the underlying — the directional sensitivity.
Delta measures an option's price sensitivity to a $1 move in the underlying, ranging from 0 to 1.00 (calls) or 0 to −1.00 (puts). A 0.50 delta call gains ~$0.50 for every $1 the underlying rises. Delta also roughly approximates the market's implied probability of the option finishing in-the-money: a 0.30 delta call has about a 30% implied chance. Traders use delta to size directional exposure: buying a 0.70 delta call behaves like holding 70 shares, with a fraction of the capital at risk.
Delta translates options into stock equivalents. Once you think in deltas, options stop being exotic and start being just another way to express a directional read.
SPY at 584. Your 580 call has 0.65 delta. SPY rallies $2 → the call gains ~$1.30. You captured 65% of the move with the premium as your only risk.
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