Definition. 25 Delta Skew Normalized (6 Months) is the relative richness of put versus call implied volatility on options expiring in roughly six months, 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.

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.