suddenness
Concise Oxford English Dictionary © 2008 Oxford University Press:
sudden/ˈsʌdn/
▶adjective occurring or done quickly and unexpectedly.
– phrases
(all) of a sudden (or archaic on a sudden) suddenly.
(all) of a sudden (or archaic on a sudden) suddenly.
– derivatives
suddenly adverb,
suddenness noun.
suddenly adverb,
suddenness noun.
– origin ME: from Anglo-Norman Fr. sudein, from an alt. of L. subitaneus, from subitus ‘sudden’.
'suddenness' also found in these Oxford entries:

