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DEFINITION A BIT OF HISTORY...
THE OHM'S LAW (Source: Wikipedia Encyclopedia)

Ohm's law states that, in an electrical circuit, the current passing through a conductor from one terminal point on the conductor to another terminal point on the conductor is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two terminal points and inversely proportional to the resistance of the conductor between the two terminal points.

In mathematical terms, this is written as:

Ohm's Law
I is the current
V is the potential difference
R is a constant called the resistance
The potential difference is also known as the voltage drop, and is sometimes denoted by E or U instead of V.

 A voltage source, V, drives an electric current, ,  I, through resistor, R
The three quantities obeying Ohm's law: 
V = IR.
Georg Simon Ohm

The law was named after the physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage, and current passing through, simple electrical circuits containing various lengths of wire, and presented a slightly more complex equation than the one seen in the definition part to explain his experimental results. The equation explained in the previous definition could not exist until the ohm (a unit of resistance) was defined (1861, 1864).
ATTENTIONFor real devices (resistors, in particular), this law is usually valid over a large range of values of current and voltage, but exceeding certain limitations may result in losing simple direct proportionality.