Also known as: 夢精
In general, images with someone having or implied to have a lewd dream.
Sometimes it's as straightforward as the dream itself being depicted in a Thought Bubble, but not always. Another sign of a wet dream is a so-called "nocturnal emission" (ejaculating while Sleeping for those with a penis, Female Ejaculation or general vaginal wetness while sleeping for those with a vagina), which may result in Stained Sheets.