Skew is a term used in several different ways. The two most common ways I hear is the spread between months (although generally that is referred to as term structure). The other and more common use of the term has to do with the relation of the level of implied volatility of puts relative to the ATM options or OTM calls. Shops use several different methods to measure skew. Many use the 25 delta call relative to the 25 delta put at either 30, 60 or 90 days to expiration. Others measure puts and calls relative to ATM option (my preferred method). The CBOE SKEW Index uses the relation of far out-of-the-money puts (sometimes called tail risk puts, swan puts, or units) relative to more standard OTM puts.