The<unk>  (symbol<unk>  is a unit of energy.  It is the amount of energy equivalent to that gained by a single unbound electron when it is accelerated through an electrostatic potential difference of one volt, in vacuo.  In other words, it is equal to one volt (1 volt = 1 joule per coulomb) times the (unsigned) charge of a single electron.  The one-word spelling is the modern recommendation, although the use of the earlier<unk>  still exists.
One<unk>  is a very small amount of energy:
The unit<unk>  is accepted (but not encouraged) for use with SI. It is widely used in solid state, atomic, nuclear, and particle physics, often with prefixes m, k, M, G or T.  In a recorded lecture from 1961 Richard Feynman apologized to his students for this failure by atomic physicists to use the appropriate SI unit (which would be the attojoule):
In chemistry, it is often useful to have the molar equivalent, that is the kinetic energy that would be gained by a mole of electrons passing through a potential difference of one volt. This quantity is equal to 96.48538(2) kJ/mol. Ionization energies and other atomic properties are often quoted in<unk> s, especially in older texts.
Electronvolt
Liter
Nanometre
Molar 
Idose
Cubic meter
Meter
Molecular orbital