What is resistance?

Resistance is the opposition to flow or movement.


Resistance in a a mechanical system could be friction, wind, gravity.

Resistance can be opposition to a policy, or to an invasion (e.g. French Resistance).
Resistance in an electrical circuit is the limiting or opposition to current flow. It is measured in Ohms.

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Electrical Resistance

The technical definition is "Electrical resistance is a ratio of the degree to which an object opposes an electric current through it, measured in ohms."
In simplfied terms, electrical resistance is broadly equivalent to friction in a mechanical system. If you applied a voltage to a circuit with zero resistance then you would get an infinite current. In reality all circuits have some resistance which limits the current.
We can predict the current in a given circuit using Ohms law:
V = I x R
Where:
V is Voltage (measured in Volts)
I is current (measured in Amps)
R is resistance (measured in Ohms)
You will often see this in the form E = I x R. It is exactly the same thing, just a different convention where "E" is used for voltage instead of "V"

Ohms law is:

V=I/R
V=Voltage (volts)
I=Current (amps)
R=Resistance (Ohms)

Resistance in an AC circuit is called Impedance (Z)
Electrical resistance is the amount that an object opposes electrical current. All things being equal (temperature, purity, etc.), resistance is a function of both its physical geometry and the resistivity of the material it is made from. Impurities in materials and fluctuation of temperature can effect the amount of resistance a material will exhibit.
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Electrical resistance is the amount that an object opposes electrical current. All things being equal (temperature, purity, etc.), resistance is a function of both its physical geometry and the resistivity of the material it is made from. Impurities in materials and fluctuation of temperature can effect the amount of resistance a material will exhibit.

There are many different types of resistance, and I recommend that for starters you read up on the Wikipedia article on electrical resistance.

http://en.wikipedia.org/wiki/Electrical_resistance