The difference in implied volatility when various options are compared with different expiration dates. This refers to the increase or decrease in implied volatility when given strike price, and a range of expiration dates. Horizontal skews can be described as "forward" or "reverse", where forward skew has increased volatility the further in the future you go, and reverse skew has decreased volatility the further in the future you go. Forward skew is by far more common, with reverse skew usually occurring as a result of news events. In the long term, volatility will tend to go up (or forward) due to implied uncertainty the further in the future you go.