Lime
In Fanfic lingo, Lime refers to fics that hints at explicit sexual activities, but does not actually show any; think "softcore" vs. "hardcore". Where Lemon would be the equivalent of an "NC-17" or "X" film, Lime is a soft "R". Lemon is the Spice Channel, whereas Lime is more like the Red Shoe Diaries.
Also known as "artificial lemon flavouring".
Generally the sexual activity displayed on Limes may simply be heavy-duty making out that jumps to the morning after, or in some cases sex described very mildly - not necessarily IKEA Erotica, although it's an option, but close. Most Lemon writers start off with Lime fics and work their way up, frequently one-handed.
Like most of the Citrus Scale, the term faded away from disuse in the early- to mid-2000s, but returned to fandom's collective consciousness after the Tumblr Kiddening of 2018.
As a bit of trivia, it's worth noting that limes are not less sour than lemons. However, they do contain less Vitamin C.