A very vertical wave face where the crest or lip of the wave throws forward to generate a tube or barrel. In addition, applied to illustrate a swell which is nearing a forecast position at a very sharp angle in comparison to the path the coastline faces. The inbound swell energy may be just on the fringe of the swell window, which normally means only the very best exposed breaks to the incoming swell will be capable of picking it up. May be used as a term by the National Data Buoy Center (NDBC) to summarize the prominent sea state at a buoy. For a given wave height, steep waves signify a more serious threat to capsizing vessels or harming marine structures than a broad swell.