まんいち万一man'ichiばんいち万一ban'ichiまんいつ万一man'itsuばんいつ万一ban'itsu
1.

A rare occurrence or event that is highly improbable but may happen under unforeseen circumstances, often used to signify the need for preparation for such an outcome.

2.

An expression used to contemplate a situation that is unlikely to occur, usually with an implication of an undesirable or negative consequence, signaling a hypothetical scenario.

3.

A consideration of an extremely low probability event, indicating a sense of caution and preparedness for an unexpected mishap.

Example:

万一の場合に備えて貯金するのは賢明だ。