A sneak attack is a type of attack used by several characters in the Dragon Ball series. It is used to deliver severe damage to off guard opponents.
Sneak Attacks are a type of attack aimed at catching a foe off guard, and are usually done from behind an opponent, when the opponent has their guard down, or to stealthily attack someone who is unaware that the attacker is even present.
Sneak attacks are some of the most lethal attacks in the series, as if the victim does not see them coming, they could take severe damage from the attack - even if they possess a power level much greater than the attacker. This makes sneak attacks the most effective attacks in the series.
Notable examples of the effectivness of sneak attacks are when it was said that Goku (with a power level of 5,000) could take down Recoome (who had a power level of around 40,000) in a single blow, when Vegeta (who had a power level around 250,000) thought he could kill second form Frieza (who possessed a power level over 1,000,000) with a single energy wave (though Frieza had his guard up so the attack failed), and when Sorbet's ray gun was able to take down Goku (who was in the Super Saiyan God SS form) in one blast.
In order to defeat Monster Form Zarbon, Vegeta throws dirt into his face, catching the general off guard and allowing Vegeta to deal a rapid blow of attacks to him - despite Monster Zarbon being superior to Vegeta power wise, he was greatly injured by the attack and his strength dropped. Vegeta later used a sneak attack on Recoome - when the Ginyu Force member thought he was unconscious, Vegeta blasted him directly in the face with an energy wave, doing a great amount of damage and knocking him back. When Recoome attempted to finish Vegeta with his Recoome Eraser Gun, Krillin attacked him with an unexpected kick to the head, causing Recoome to shut his mouth quickly and doing a great amount of damage to him. Later, while Recoome had his guard down and was preparing his Recoome Fighting Bomber, he was heavily hit in the stomach by Goku and knocked out, Burter then stated that the only reason that Recoome was defeated was because he was hit with his guard down - showing that even though they believed Goku had a power level of 5,000, even this low power level would be enough to defeat Recoome (who possessed a power level of 40,000) in one blow with an unexpected attack.
Captain Ginyu later attempted to catch Vegeta off guard using his Body Change technique, it almost worked, however Goku managed to stop the attack by throwing a Namekian Frog in-between the attack and Vegeta.
During the battle against second form Frieza, while Frieza had his back turned Vegeta attempted to kill him with an energy blast, however Frieza had his guard up as he saw the move coming, and was unaffected by the attack.
Later during the battle with Mecha Frieza, Super Saiyan Future Trunks caught the tyrant off guard by attacking him with a blast that was not intended to hit him, allowing the Saiyan-Human hybrid to attack Frieza from above. King Cold then attempted to attack Future Trunks with a quick blow, however Trunks saw the attack coming and blocked it.
Upon being revived in his Super Perfect form, Cell attacked Future Trunks with a Full Power Death Beam while he has his guard down, instantly killing him. During the final beam struggle between Super Saiyan 2 Gohan and Super Perfect Cell, Vegeta managed to hit Cell with an unexpected attack, catching the Bio-Android off guard and allowing Gohan to finish him off.
In the movie Dragon Ball Z: Bojack Unbound, when Full Power Bojack had Gohan in a bear hug, base Goku used his Instant Transmission technique in order to hit Bojack with a surprise attack, damaging him while also knocking him far away and causing him to release Gohan.
In the movie Dragon Ball Z: Resurrection ‘F’, in order to demonstrate how letheal having your guard down was, Whis attacked Goku with a blow to the side when Goku believed they were merely talking - causing incredible pain to the Saiyan. Later, when Super Saiyan God SS Goku had his guard down during his battle with Frieza, Golden Frieza had Sorbet shoot Goku with his ray gun, dealing critical damage to the godly Saiyan.
During the Baby Saga, base Trunks managed to knock Golden Great Ape Baby off his feet by hitting him with a surprise energy blast. Later, in the finale of the Shadow Dragon Saga, base Trunks was able to send Omega Shenron flying by hitting him with a kick he did not see coming.