Question

What is a domain?

Answer 1

Given a function f: $X\to Y$, the set X is the domain of f; the set Y is the codomain of f. In the expression f(x), x is the argument and f(x) is the value.
In simple words we can say that domain of any function is the value for which the function is defined.

For example: Domain of sin x is set of all real numbers as sine function is defined for all real numbers.