What is number system?

Answer 1

A number system is a set of rules and symbols to express an amount. Depending on the number system the same amount is expressed very differently.
Nowadays, we humans use the Arabian decimal number system in which a digit is to the base of 10.
The Romans have been using a number system that is not exponential but additive.
Computers use a number system to the base of 2.

Answer 2

A system for representing number of a certain type

Answer 3

*numbers

Answer 4

Or this may be a collection of things (usually called numbers) together with the operations on those numbers and the proterties that the operations satisfy