Delta
Change in option price per $1 move in the underlying — ranges 0 to 1 for calls, -1 to 0 for puts.
In depth
A 0.5 delta call gains $0.50 per $1 stock rise. Delta also approximates probability of finishing in-the-money. Deep ITM options have delta near 1; deep OTM near 0. Delta-hedged positions try to neutralize directional exposure.