na-adjective
sudden; unexpected; unforeseen
Describes events, actions, or changes that occur abruptly and without warning. Often used in formal or written contexts.
突発的な停電で会議が中断された。
The meeting was interrupted by a sudden power outage.
People around him were surprised by his unexpected behavior.
不意 is a noun meaning 'unexpectedness' and is often used in set phrases like 不意に (unexpectedly), whereas 突発的 is an adjectival noun describing the nature of an occurrence.
Compound of 突発 (sudden outbreak) + 的 (-like, -ish). 突発 is from 突 (thrust, suddenly) + 発 (occur, happen).