Idempotence

The word idempotent was first used in 1870. It comes from the Latin words idem "the same" and potentem "powerful". It is really only encountered in the realms of mathematics and in computer science.

Put (very!!) simply, it means that further callings upon a state or variable are not necessary as one instigation has the same outcome as multiple ones.

This is (for my way of thinking as a physicist) best thought of in terms of logic gates.

. =	Here we have and as inputs into an AND gate. Both inputs have to high for the output to be high... and as they are identical the output will depend on the state of alone... the gate's presence is therefore pointless!
+ =	Here we have and as inputs into an OR gate. If either input is high the gate output will be high... and as they are identical the output will depend on the state of alone... the gate's presence is therefore pointless!