na-adjective, noun which may take the genitive case particle 'no', noun
sudden; unexpected; unforeseen
Describes something happening without warning or anticipation. Often used in adverbial phrases like 不意に (suddenly) or in compounds such as 不意打ち (surprise attack).
I was surprised by the unexpected visitor.
He suddenly stood up.
I was surprised by an unexpected visitor.
I was caught off guard when someone called out to me from behind.