A body is in balance when the net some of the forces on it is zero.
For a dancer with respect to gravity, this can occur only when the center of mass of the body is located over the standing foot or between standing feet.
Clearly this is not preserved in dancing - all good dancers can often be seen projecting their weight well oustide their footprints.
Some have said that the above is "static balance" while the thing that must be preserved is "dynamic balance". I would challenge them to come up with a strict definition of "dynamic balance" which can satisfy two real-world constraints based on how the term "balance" is used by dancers:
1) It must be obviously preserved by good dancers
2) It must be easy to propose situations where less skilled dancers would loose it.
In reality, I believe that when dancers use the word "balance" what they really mean is the never letting gravity take you somewhere you didn't want to be going or at a rate at which you didn't want to be moving. The important point being that you are often off balance and gravity is moving and accelerating you - but only in the ways you wanted it to, because you aimed your movement taking the contribution of gravity instinctively* into account.
(*instinctively for a trained dancer)
