Overdrive Monsters (オーバードライブモンスター, Ōbādoraibu Monsutā) are a type of Extra Deck monster used in Yu-Gi-Oh! Xross. They are special summoned to a Drive Zone and a monster in said zone. Their border is a Dark Red lightning pattern.
Overdrive Monsters do nor posses Levels, Ranks or any indicator shared by another type of monster and are unaffected by effects that would target them. Instead, they have a "Limit": which is equal to the difference between the Drive Zone's "Drive" and the level of the other Overdrive material monster. Limits are depicted by a row of yellow circles with red stars in them that extend outwards to the right-hand side of the card (much like levels).
To perform an Overdrive Summon, a "Driver Monster" must be set face-up and upside-down in a Main Monster Zone, which becomes a "Drive Zone" with a value called a "Driver" (this may be done as many times as possible). The Driver is treated as a zone, and is affected by effects that affect zones. By sending a monster in a Drive Zone and the Driver to the GY, an Overdrive Monster with a Limit equal to the difference between the Drive Zone's Drive and the monster's level may be Overdrive Summoned.