also: ふしんか
noun
suspicious fire; suspected arson
Used in news reports and official contexts for a fire whose cause is unclear and possibly deliberate.
昨夜、駅前の空き家で不審火があった。
Last night there was a suspicious fire at an empty house in front of the station.
警察は連続不審火の関
The police are investigating a possible link between the series of suspicious fires.
放火 is the criminal act of arson, while 不審火 refers to a fire itself that is suspicious and may have been set deliberately.
Compound of 不審 (suspicious, questionable) and 火 (fire).