A solenoid is a device consisting of an electric wire wound evenly in a helix to form a long coil. When a current passes through it the solenoid produces a magnetic field in its vicinity and more especially inside the coil where this field is virtually uniform.
For a very long solenoid, wound with n turns per unit of length, carrying a current I, the magnetic induction inside is practically uniform with an intensity given by the relation B = µ0.n.I.
Modelling of a solenoid. credits: Yves Pelletier.