Mimicry refers to the coveted ability to copy another fighter's technique after seeing it used only once.
It is first used by Goku when seeing Master Roshi perform a Kamehameha to extinguish Fire Mountain in Dragon Ball. Goku later uses it at the 21st World Martial Arts Tournament, picking up Master Roshi's Afterimage and Drunken Boxing techniques, but modifying them to suit his own purposes. It is also used by Tien Shinhan during his quarter-final match with Yamcha during the 22nd World Martial Arts Tournament: Yamcha fires a powerful Kamehameha at Tien only to have it nullified and reflected by Tien's Kiai blast. Tien then later surprises everyone by firing his own Kamehameha at [Jackie Chun during their semi-final match. During the final match, Goku develops King Chappa's Hasshu-ken technique in retaliation against Tien's Four Witches Technique.
In the Vegeta Saga, Chiaotzu is also shown to be able to use the Mimicry ability: after witnessing what the Saibaman's Self Destruction technique did to Yamcha earlier in the battle, Chiaotzu decides to copy it himself and later uses his version of the technique (named Farewell, Mr. Tien) against Nappa, to no avail however. Vegeta copies Krillin's Destructo Disc, using it to cut off Gohan's tail while the latter is in Great Ape form. Vegeta also managed to learn Ki Sense after his fight against the Dragon Team.
In his 100% Power form, Frieza uses a variation of Krillin's Destructo Disk technique called the Death Saucer in an attempt to kill Goku. He witnessed the technique while in his second form when Krillin cut off his tail. Frieza also uses it in Dragon Ball GT, where Goku says he "stole it from Krillin".
Both Majin Buu and Kid Buu are able to copy the Super Kamehameha after only seeing it used once though it results in the development of the Majin Kamehameha. According to Goku, Majin Buu also copied Vegeta's Continuous Energy Bullet. Kid Buu also copies Kibito Kai's Kai Kai after seeing it only once.
Gotenks' Bam Bam technique is to copy another person's technique. It is used to copy Mr. Satan's Dynamite Kick on Super Buu. As a Super Saiyan 3, Gotenks mimics Super Buu's Vice Shout in order to rip dimensions so that he and Piccolo can escape the Hyperbolic Time Chamber.
Goku Black is shown to be able to use this ability, but unlike other users, Goku Black's ability to use mimicry advances his power and gives him better access to his power, as he himself noted when he copied Goku's stance and attacks, he became more powerful every time he did so. He appears to be the only one who uniquely become more powerful with this ability as he continues to use it.
In Dragon Ball: Yo! Son Goku and His Friends Return!!, Gotenks copied Yamcha's Wolf Fang Fist and Tien's Volleyball Fist to use on Aka.
In Dream 9 Toriko & One Piece & Dragon Ball Z Super Collaboration Special, in his Fifth Form: Super Big Tuna, Akami has the ability to mimic attacks. The monster uses this to copy all of the attacks that Goku, Luffy, and Toriko had used against it while it was in its previous form.
Video Game Appearances
Majin Ozotto, the final boss of Dragon Ball Z: V.R.V.S., has the power to shapeshift into the Dragon Team in the game and use their techniques. Mimicry is Majin Buu's special ability in Dragon Ball: Zenkai Battle: after seeing an opponent's long-distance energy attack, he can use "Kamehameha" to replicate it, and after seeing a close-range physical technique, he can replicate it using "Buu Rush".
In Dragon Ball Xenoverse 2, the Future Warrior uses mimicry in the same method as the original Future Warrior to learn techniques. Additionally due to Broly's brutal insanity he does not actually teach the Future Warrior his skills as the Future Warrior uses mimicry to learn his skills, which Broly himself notes while "sparring" with them. Future Zamasu also mentions the Future Warrior copying some of his techniques (Future Zamasu however allows them to do so as part of their training).
In Dragon Ball Fusions, Mimicry appears as a Special Move that allows users to copy and use one of their opponent's Special Moves regardless of its nature (for example they can even mimic weapon-based techniques or techniques that would normally require cybernetic modifications). Additionally during the main story, Nappa and Raditz use mimicry to learn the Fusion Dance after witnessing it being performed by Goten and Trunks.
In Dragon Ball FighterZ, Android 21 and Android 21 (Evil) can both use mimicry like Majin Buu due to her Majin cells. In the Super Warrior Arc, Android 21 uses it to learn Instant Transmission by copying Goku after he uses it to transport himself and his allies to the Sacred World of the Kais. In the Android 21 Arc, Evil Android 21 (Cell Absorbed) "snags" Instant Transmission from Goku when he uses the technique to get Senzu Beans from Korin and return to Dr. Gero's Secret Lab to give them to Android 18, Android 17, and Android 21 (Good). Android 21 flees using the technique as she is not ready to feast them. It is unknown if Android 21 (Good) can utilize this technique as she is never see using it.