Definition. 25 Delta Skew Normalized (All) is the relative richness of put versus call implied volatility on options, computed as the difference between a 25-delta put's implied volatility and a 25-delta call's implied volatility, normalized by the at-the-money implied volatility. The individual series cover option contracts expiring 1 week, 1 month, 3 months, and 6 months from now.

Technical. A 25-delta put has a delta of -25% and a 25-delta call has a delta of 25%, sampling the option surface at symmetric points either side of the money to expose the put-versus-call asymmetry in implied volatility.

Interpretation. Negative readings mean calls trade at a higher implied volatility than puts. Positive readings mean puts trade at a higher implied volatility than calls.