Thunderclap, Thunderbolt, and Thunderhead are a series of Jupiter attack Psynergies featured in both Golden Sun games. It is available exclusively to the Ninja class series. Hence, all of the Adepts in your party, except for Mia and Piers, can use this series of Psynergy.
Thunderhead was originally called Thunderstorm, but this name was too long for the single line alloted to it, causing some display glitches that sometimes occur when it is highlighted. This problem was fixed in Golden Sun: The Lost Age by changing "Thunderstorm" to the just-barely-shorter "Thunderhead".
Thunderclap is the first spell of the series. It is described as an "attack with the storm's fury." It appears as a single, wide bolt of purplish-white lightning firing down on the enemy before exploding on impact with the ground. Thunderclap is learned at Level 8, hits three targets with Jupiter power of 50, and costs 9 PP to perform.
Thunderbolt is the second ability learned. It is described as an "attack with the storm's fury." It takes on the form of three individual Thunderclap attacks striking in succession. Thunderbolt is learned at Level 26, hits five targets with Jupiter power of 110, and costs 19 PP to perform.
Thunderhead (Thunderstorm in Golden Sun) is the third and final power in the series. It is also described as an "attack with the storm's fury." It appears as a barrage of normal Thunderclap attacks being volleyed without ceasing into the targeted enemies. Thunderhead is learned at Level 50 and it hits seven targets for Jupiter power of 190, and costs a tremendous 39 PP to perform; it is, in fact, the most expensive Psynergy in the Golden Sun series, tied with Reigning Dragon from Golden Sun: Dark Dawn.
Unlike the other two Psynergy series' exclusive to the Ninja class and with counterparts in other classes, Thunderclap is actually stronger than the Psynergy it is similar to, Plasma. The Plasma series is considered one of the strongest Psynergy in the game, and the Thunderclap series is actually better. The only potential pitfalls to this is the high levels at which they are learned and the fact that they require a lot of Psynergy, the most of any Psynergy in either game. This is offset by the large pool of Psynergy that a Ninja receives, with the Master class receiving a 160% bonus.