What does Delta tell us?

Delta tells us how much something has changed, how much difference there is between two states, or how sensitive one variable is to another, depending on the field. In general math and science, delta, written as Δ, tells us the absolute change. For example, ΔT tells you how much temperature changed, and Δv tells you change in velocity. It quantifies difference. In finance and options trading, delta tells us the most important risk measure. It tells you how much the price of an option is expected to move for a $1 move in the underlying stock. A delta of 0.50 means if the stock moves $1, the option should move about $0.50. It also tells you the approximate probability of an option expiring in the money. A 0.30 delta call has roughly a 30 percent chance of expiring in the money. In chemistry, delta tells us about partial charges in polar molecules, showing where electrons are more concentrated. In geography, a river delta tells us where sediment has been deposited over thousands of years where a river meets the sea, indicating fertile land. In business, delta often means the gap between current performance and target performance. So overall, delta is a universal shorthand for telling us about change, difference, sensitivity, and distance from a goal.