Want a more technical definition? Wikipedia says: often used slang term in the Japanese language for playfully sexual actions….ecchi has been adopted by western fans of Japanese media to describe works with sexual overtones. In Japanese, the word ecchi is often used to describe a person’s conduct, but in western fandom, it has come to be used to refer to softcore or playful sexuality, as distinct from the word hentai, which connotes perversion or fetishism.
Give the wiki a read, its actually pretty fascinating to be honest, especially when it discusses the western and eastern uses of words like ecchi and hentai and how they differentiate.