A V-twin engine is a two-cylinder internal combustion engine where the cylinders are arranged in a V configuration.
Gottlieb Daimler built a V-twin engine in 1889. It was used as a stationary powerplant and to power boats. It was also used in Daimler's second automobile, the 1889 Stahlrad ("steel-wheel"). The engine was also manufactured under licence in France by Panhard et Levassor. In 1903, both Glenn Curtiss in the United States and NSU in Germany began building V-twin engines for use in their respective motorcycles. Peugeot, which had used Panhard-built Daimler V-twins in their first cars, made its own V-twin engines in the early 20th century. A Norton motorcycle powered by a Peugeot V-twin engine won the first Isle of Man Tourist Trophy twin-cylinder race in 1907.
Most V-twin engines have a single crankpin, which is shared by both connecting rods. The connecting rods may sit side-by-side with offset cylinders, or they may be "fork & blade" items with cylinders in the same plane without an offset.
Some notable exceptions include the Moto Guzzi 500cc (with 120° V angle and 180° crank pin offset) that Stanley Woods rode to win the 1935 Isle of Man TT; the 1983 Honda Shadow 750, claimed as being the first V-twin with an offset-dual-pin crankshaft; and the 1987 Suzuki VX 800, 45° V angle with 45° crank pin offset in USA and 75° crank pin offset for the rest of the world.
Generally, any two-cylinder motorcycle engine with its two cylinders arranged more than 0° and less than 180° apart is referred to as a V-twin. Although Ducati use the name "L-twin" for their 90° twin engine (with its front cylinder nearly horizontal and the rear cylinder almost vertical), there is no technical distinction between V-twin and L-twin engines; and these are merely names used by convention.
A 90° V-twin will, with a correct counterweight, yield perfect primary and secondary balance, although its firing intervals are uneven. A V-twin with an angle of less than 90° is more compact and has more even firing intervals, but has significantly poorer mechanical balance. Offset crankpins are sometimes used to reduce the resulting vibration.
The terms longitudinal engine and transverse engine are most often used to refer to the crankshaft orientation, however, some sources, most prominently Moto Guzzi, use the terminology in the opposite way.
A Moto Guzzi Technical Services representative tried to explain to LA Times columnist Susan Carpenter that Moto Guzzi engines are "called 'transverse' because the engine is mounted with the crankshaft oriented front to back instead of left to right." In spite of this, it is could be assumed that those who call V-twin motorcycle engines "transverse" when they are mounted with the crankshaft front-to-back and the cylinders sticking out the sides are saying that to them, the engine's axis is the line passing from one cylinder to the other, at a right angle to the crankshaft, rather than going by the crankshaft's axis. Highly technical sources, such as V. Cossleter's Motorcycle Dynamics, or Gaetaeno Cocco's Motorcycle Design and Technology are careful not simply to use the terms "longitudinal engine" or "transverse engine," but rather to specify that they mark the engine's orientation based on the crankshaft, and so they will say "transverse crankshaft engine" or "longitudinal crankshaft engine", or, conversely, "transversely mounted cylinders" in referenced to the classic BMW orientation, with a longitudinal crankshaft and cylinders at a right angle to the axis of the frame.