Delta, or how many ES to hold

The useful definition

An option's delta is the change in its price for a 1-point change in the index. An at-the-money call has a delta close to +0.50; an at-the-money put, close to −0.50. Far from the money, deltas tend toward 0; deep in the money, toward +1 or −1.

For a flow trader, delta has one translation only: how many ES contracts the market maker has to hold to be flat. Since an SPX option is two ES in notional, a delta of 0.50 on an option sold requires a hedge of 1 ES.