A unified transition is one in which both feet are used simultaneously at some point in the transition. This can be done as a either a landing on both feet, a takeoff from both feet, or both. 3 of the 4 unified transitions, the pop, punch, and bound, are extremely common, and often feel quite natural to the beginner and professional alike. The least utilized, the reverse-pop is itself a powerful transition, when used properly, but sometimes require a bit more finesse and experience to fully utilize its potential.
Any sequential landing followed by a two-foot takeoff from both feet simultaneously.
Unified landing followed by a unified takeoff. (2 feet -> 2 feet)
Unified landing followed by a sequential takeoff.
Any unified transition (pop, punch, or reverse pop) + adjustment step + unified takeoff.