Blank

Lacking something essential to fulfillment or completeness; unrestricted or open. A space left empty for the insertion of one or more words or marks in a written document that will effectuate its meaning or make it legally operative. A printed legal form in which the standard or necessary words are printed in their proper order with spaces left open, to be filled with names, dates, figures, and additional clauses.

A blank check is one that is unrestricted as to the amount to be paid.