The ohm is the SI unit of electrical resistance. Its symbol is the Greek capital letter omega (Ω). The ohm is named for Georg Ohm, a German physicist who discovered the relation between voltage and current, expressed in Ohm's Law.
By definition in Ohm's Law, 1 ohm equals 1 volt divided by 1 ampere. In other words, a device has a resistance of 1 ohm if a voltage of 1 volt will cause a current of 1 ampere to flow.
A thousand ohms is called a kilohm (not kilo-ohm). A million ohms is called a megohm (not mega-ohm).
A measurement in ohms is the reciprocal of a measurement in siemens (also called mho), the SI unit of electrical conductance. Note that 'siemens' is both singular and plural.