What is the proper definition of 'head of household'?

Are you the 'head of household' if you are single with no dependants and working one job?

No - that makes you single.

Head of Household means that you are unmarried, paid more than half the cost of the home you live in, and have a "qualifying person" living with you - for example, if you are divorced and your child lives with you, or a relative such as a sibling or grand parent lives with youand is your dependent, or you have a parent who is a dependent even if they aren't living with you. The full definition is here:

http://www.irs.gov/pub/irs-pdf/p501.pdf

Ebaines answers this question well.

I have no additional input!