1.a particular environment or walk of life;
his social sphere is limited
2.territory over which rule or control is exercised;
his domain extended into Europe
3.the set of values of the independent variable for which a function is defined
4.people in general; especially a distinctive group of people with some shared interest;
the Western world
5.a knowledge domain that you are interested in or are communicating about;
it was a limited domain of discourse