What is Resistance ?(units of resistance) (ohm definition)

Resistance(R)

Resistance may be defined as that property of a substance which opposes (or restricts) the flow of an electric current (or electrons) through it.

Units and ohm definition

The practical as well as mks (or SI) unit of resistance is ohm (Ω), which is defined as that resistance between two points of a conductor when a potential difference of one volt, applied between these points, produces in this conductor a current of one ampere, the conductor not being a source of any emf.

For insulators having high  resistance , much bigger units kilo ohm or kΩ (10^3 ohm) are used. in case of very small resistances smaller units like milli ohm (10^-3) are employed.

Relation Between electric conductivity and  resistance :

Electric Conductivity  and  Resistance are inversely proportional to each other. the reciprocal of resistance is called conductance.

Conductance = 1/R

For More Detailed Explanation see the below video.

See Also:

Resistance and it's units

Laws of Resistance

Ohm's Law and it's limitations

Conductance and Conductivity

Current Density

Types of Current Supply

Conductors insulators and semiconductors