Retromimesis (also known as retromimicry) is a type of spell that was used by Papa Smurf on Gargamel in the comic book story "Smurf Vs. Smurf" (and also in the cartoon show episode "Romeo And Smurfette", though in that story it didn't have an actual name) that caused the both of them to exchange appearances. The spell required both of the participants to lock eyes on each other while the spell caster recites the spell to activate the transformation.
In the stories that this spell appeared in, Papa Smurf used it so that he could get his little Smurfs to stop fighting each other by having them turn their attention on Gargamel instead, with Papa Smurf being in the appearance of Gargamel. Meanwhile, Gargamel who was in the appearance of Papa Smurf sought after the counter-spell that would restore their normal appearances, and found it just in time as Papa Smurf in the appearance of Gargamel was staked down by the Smurfs. Once the spell was reversed, Gargamel tried to take advantage of the situation by chasing after the Smurfs, but was attacked by his cat Azrael who still assumed that Gargamel was actually a Smurf. After losing the Smurfs, Gargamel tried to retrace his steps to return to the Smurf Village, but instead ended up back at his own hovel.
The name of the spell comes from the original Random House English adaptation of "Smurf Vs. Smurf", known as "Smurf Of One And Smurf A Dozen Of The Other".