"Lemons are stories that explicitly depict sexual activities, so named because they leave a sour taste in one's mouth, if not other places. A lime is a lemon with most of the really good bits taken out."
"A Lemon-fic doesn't need to be vulgar. It just needs to make you check your parents aren't in the house. And a good
Lemon-fic just needs to make you want to read the next chapter. Except, one-handed
"Now I know why they call 'em lemons. They leave a sour taste in my mouth."
— from Anime Detective: Crossover Crisis, by Pearson "Doc" Mui.