Question

what is a domain?

Answer 1

In terms of math, a domain is the set of all possible values of x that a function can be. For example, with the equation: \[y=x ^{2}\]
The domain is the set of all real numbers, because there are no limits on what x is allowed to be. With the function....
\[y=\sqrt{x}\]
The domain is the set of all numbers greater or equal to zero, because it is not possible for x to be less than 0.

Hope this helps. Here's a more detailed explanation...
http://en.wikipedia.org/wiki/Domain_of_a_function