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.