A V-twin engine is a two-cylinder internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 where the cylinders are arranged in a V

V engine

A V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...

 configuration.
Most V-twin engines have a single crankpin, which is shared by both connecting rod

Connecting rod

In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....

s. The connecting rods may sit side-by-side (so the cylinders are offset), or they may be "fork & blade" items which have the same plane (whereby the cylinders are not offset).

Some notable exceptions include the Moto Guzzi

Moto Guzzi

Moto Guzzi is an Italian motorcycle manufacturer. It is one of seven brands owned by Piaggio.Established in 1921 in Mandello del Lario, Italy, the company is noted for its central historic role in Italy's motorcycling manufacture, its prominence worldwide in motorcycle racing, and a series of...

 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

Honda Shadow

The Honda Shadow refers to a family of cruiser-type motorcycles made by Honda since 1983. The Shadow line features motorcycles with a liquid-cooled 52-degree V-twin ranging from 125cc to 1800cc engine displacement...

 750, claimed as being the first V-twin with an offset-dual-pin crankshaft; and the 1987 Suzuki VX 800

Suzuki VX 800

The Suzuki VX800 is a road motorcycle shaft-drive V-twin bike from Suzuki.The VX800 was designed at U.S. Suzuki's Design Studio in Brea, California from 1986 to 1989, and produced in years 1990 to 1997...

, 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.