What is the definition of non-sufficient funds?

Not if you wrote the bad check....   In business, probably always, as some type of business expense.On personal tax, probably never, unless you could possibly tie it to an investment or...

Probably if you knew or should have known about the fee before the NSF happened. If you could not know then you may have a basis for dispute.

You can contract a collection agency to recover it, or you can take it to the prosecutor's office and file criminal charges against the person who wrote it.Your best bet it to take it to collections....

A due payment which has NOT been paid!

A private investment vehicle whose manager receives a significant portion of its compensation from incentive fees tied to the fund's performance -- typically 20% of annual gains over a certain hurdle...