Accel Synchro Monster is a term used by the anime to describe Synchro Monsters that can only be Summoned by using 1 Synchro Tuner Monsters and 1 or more non-tuner Synchro Monsters; this type of Synchro Summon is referred to as an "Accel Synchro Summon" in the anime. Accel Synchro Monsters first appeared on Yu-Gi-Oh! 5D's - Episode 074, being used by the duelist Bruno who was known at that time as Vizor (Dark Glass in Japan).
Performing an Accel Synchro Summon is treated as Spell Speed 2, as Vizor is able to chain his Accel Synchro Summon to Yusei's activation of a card effect (on Yusei's turn). This, coupled with the individual effects of the Accel Synchro Monsters, provide a defense against effects that target Synchro Monsters, such as but not limited to the Meklord monsters, leading Yusei and the rest of Team 5D's to pursue the secrets behind Accel Synchro Summoning during the events leading up to, and during, the World Racing Grand Prix.
Delta Accel Synchro Summon is a form of Synchro Summoning, in which requires 1 Synchro Tuner Monster and at least 2 or more non-Tuner Synchro Monsters such as "T.G. Halberd Cannon". It was used by Bruno Antinomy, when performing Top Clear Mind in Yu-Gi-Oh! 5D's - Episode 144.
In the anime, in episode 74, it is hinted that Accel Synchro Monsters have other unknown abilities, due to Vizor saying he is not as good a person up to the point of revealing all the secrets of Accel Synchro Monsters.
Accel Synchro Monsters premiered in the Booster Pack Starstrike Blast with Shooting Star Dragon, the set's Cover Card, as well as the first of the printed Tuner Synchro Monsters, Formula Synchron.