In essence, delta is a measure of how long or short the holder of an option position is in terms of the underlying asset. A hard delta (usually above 90) refers to a delta hedge which is implemented using options, and whereby delta doesn’t vanish asymptotically to the underlying asset price.
Hard deltas result from at-the-money (ATM) or in-the-money (ITM) options. Because an ITM option with a hard delta almost replicates the behavior of the underlying stock, hard delta is much less affected by external factors or occurrences compared with options with soft delta.
