It took him several months to adapt to civilian life after retiring from the military.

civilian 是名词，由词根 civil（公民的、民间的）加上后缀 -ian（表示"与...相关的人"）构成，指"非军事人员、平民百姓"。在军事语境中特指"非军职人员"，与 military（军事的）相对。