A small, mischievous sprite or a malevolent supernatural creature, somewhat comparable to a demon but smaller and less powerful, formerly regarded as the child of the devil or a demon (see sense 3.2).
"The biochemist added imp to the solution to initiate the metabolic reaction."
Initialism of inosine monophosphate.
In plain English: An imp is a small, mischievous fairy-like creature from folklore that likes to play tricks on people.
"The kids ran out to catch the mischievous imp chasing them through the garden."
To engraft or plant (a plant or part of one, a sapling, etc.).
"The gardener carefully imp the young oak sapling into the prepared soil to ensure it takes root."
In plain English: To imp someone means to trick them into doing something they didn't mean to do, usually by pretending you are innocent when you aren't.
"The manager decided to imp all employees who arrived late for work."
Usage: Use the verb "imp" specifically when referring to planting a graft onto a tree branch. Do not use it for supernatural creatures; instead, refer to those as nouns meaning small mischievous sprites.
The word "imp" originally meant to plant or graft something in Middle English. It traveled into modern usage as a mischievous sprite through an unrelated shift from the Old Norse term for a goblin.