WordNet sense 1 (a part added to a book or play that continues and extends it):

sequel

WordNet sense 2 (a Gestalt principle of organization holding that there is an innate tendency to perceive a line as continuing its established direction):

good continuation, law of continuation

WordNet sense 3 (the consequence of being lengthened in duration):

lengthiness, prolongation, protraction

WordNet sense 4 (the act of continuing an activity without interruption):

continuance