An element of one of several sets: natural numbers, integers, rational numbers, real numbers, complex numbers, and sometimes extensions such as hypercomplex numbers, etc.
(Followed by a numeral; used attributively) Indicating the position of something in a list or sequence. Abbreviations: No or No., no or no. (in each case, sometimes written with a superscript "o", like Nº or №). The symbol "#" is also used in this manner.