Just my OP.
ecchi - No visible women nipples or anus and genitalia of either gender. Maybe cameltoe, poking-up tits under clothes, and bulging can be acceptable ecchi. It is called "ecchi" after all. (Used in sentence: "Not sure exactly how it looked, the details were ecchi.)
Hentai - Every part of the body can be exposed and captured in every position and usage.