An ohm is a unit used in the international system of measurements to measure resistance, represented by the Greek letter omega (Ω). By definition, it is equal to one volt creating one ampere in a device, abbreviated as V/A. An ampere, or amp, is a measure of current equal to a specific amount of force between a theoretical pair of infinite conductors in a vacuum.
Georg Ohm was a German physicist in the early 19th century. He was well-trained in both mathematics and physics, and held a number of distinguished teaching positions. In 1827, he published what is viewed by most people as his master work, Die galvanische Kette mathematisch bearbeitet, in which he laid out most of his general theory of electricity.
Ohm is looked at by many as one of the great fathers of electricity, as his work heavily influenced all theory of the day. Ironically, it took many years for him to gain recognition in his own life. Though he strove for a professorship at a university, he received little acknowledgment for his ground-breaking work until near the end of his life. Much of this was undoubtedly due to his reliance on mathematics as a fundamental way to describe his theories on the workings of electricity. Though much cleaner and ultimately more useful than the non-mathematical physics of the day, it was difficult for much of the establishment to accept.
The equation that yields the measurement bearing his name, Ohm's Law, is a rather simple one. It is written as V/I = R, where V is the voltage of a device, I is the current, and R is the resulting resistance. It should be noted that the resistance of a real-world device is never exactly true to this law, as impurities in the conductor and the actual behavior of electrons affect the resistance. In practice, however, such discrepancies are negligible enough to be ignored, and this law may be treated as true.
An inverse measure also exists, called alternately a siemens or a mho. It may be expressed as an inverted omega, or an ohm to the negative first power. A mho may be defined as A/V, where A is amperes and V is voltage.