At the airline level, a Captain or First Officer who is moving to the same ‘seat’ on a different aircraft is also transitioning. For example, a Captain would transition from the Boeing 737 to the Boeing 757 or the Airbus A320.
For the rest of us in General Aviation, a pilot can transition from a CE 150/152 up to a CE 172 or CE 182. A transition to a CE 182 Skylane would also require a high-performance endorsement, since the CE 182 has more than 200 hp. For a pilot flying a twin like a Baron, to move up to a cabin-class, pressurized twin Cessna, like a CE 340, it would also require transition training. Additionally, in this case, because the CE 340 has an operational service ceiling above FL 250, actually FL 300, a high-altitude endorsement is also required. Even if you never intend to fly it that high, it is required, to act as PIC of this aircraft. Refer to FAR 61.31 (g).