Zero:
Null, the arithmetical representation of nothing, as in 10(2*5)=0.
It is an interesting fact about the human mind that the idea of zero 
at least in the sense of: a special number with a place of its own in
arithmetic and algebra  only arose in the 2nd C, probably in India.
It is very useful in algebra and bookkeeping, and also has the great
virtue of making possible a positional notation for numbers, as
in 101 = one hundreds plus zero tens plus one ones.
Before the zero was thought of arithmetic was much more difficult for
lack of a positional notation (as with Roman numbers). The Ancient
Greeks did not even have a special notation for numbers, but used
letters to represent numbers.
The zero started being used in the West only in the 13th C.
