Acting alone or independently without others or without coordination; commonly used in daily speech.
彼はいつも単独行動を好むタイプだ。
He's the kind of person who always prefers to act alone.
山での単独行動は危険なので、必ず誰かに行き先を伝えてください。
Solo hiking in the mountains is dangerous, so always tell someone where you're going.
チームの仕事なのに単独行動を取ったため、混乱が生じた。
Because someone acted alone on a team task, confusion arose.
警察は容疑者の単独行動を示す証拠を発見した。
The police found evidence indicating the suspect acted alone.
プロジェクトを単独行動で進めるのは難しい場合が多い。
Often it's difficult to proceed with a project by acting alone.