na-adjective, adverb taking the 'to' particle, noun
Describes something contrary to what one thought or assumed. Often used as 意外に or 意外と (adverbially) meaning 'unexpectedly' or 'more/less than expected'. Can also be used as a na-adjective or noun.
It was surprising that he failed the exam.
This dish is unexpectedly delicious.
Surprisingly, she was already married.
I was surprised to learn an unexpected side of him.
This cake is surprisingly delicious.
This problem turned out to be surprisingly easy.
意外 means 'unexpected' in a general, sometimes pleasant sense, whereas 不慮 specifically implies an unforeseen misfortune or tragedy beyond one's control.
意外 means 'unexpected' in the sense of contrary to what one expected, while 不意 focuses on the abruptness and absence of any prior notice.
意外 is a na-adjective/noun meaning 'unexpected' and is often used as 意外に/意外と before a verb or adjective, whereas 思いのほか is itself an adverb that directly modifies the following word without a particle.
意外 is a common na-adjective/noun meaning 'surprising, unexpected' and can stand alone as a predicate, whereas 予期せぬ only modifies a following noun.