The space or region around a magnet within which it can influence other magnet or magnetic material. It is denoted by B.
Magnetic field due to current in a long straight line
Consider a current carrying long, straight wire. A magnetic field B is set up around the current carrying wire. The magnetic lines of force are concentric circular around the wire and their direction depends upon the direction of current. The magnetic field B produces when current I flows through the wire and becomes zero when the current stops. The strength of the magnetic field is greater near the wire and decreases as the distance from the wire increases.
RIGHT HAND RULE
If the wire is grasped city in the right hand with the direction of current, the thumb pointing in the direction of the magnetic field will be along the curl of fingers.