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 (Saibaman Bomb), Chiaotzu decides to copy it himself and later uses his version of the technique (named Farewell, Mr. Tien) against Nappa, to no avail however. Chiaotzu also uses a Masenko-like technique in the movie Dragon Ball Z: The Tree of Might. Vegeta copies Krillin's Destructo Disk, 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 Z Fighters.
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. According to Goku, Majin Buu also copied Vegeta's Continuous Energy Bullet. Kid Buu also copies Kibito Kai's Instantaneous Movement 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 well as Yamcha's Wolf Fang Fist and Tien's Volleyball Fist on Aka in Dragon Ball: Yo! Son Goku and His Friends Return!! 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.
In the 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.
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.
Appearances in games
Ozotto, the final boss of the arcade game Dragon Ball Z: V.R.V.S., has the power to shapeshift into the Z Fighters in the game and use their techniques. Mimicry is Majin Buu's special ability in Dragon Ball: Zenkai Battle Royale: 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".