A somewhat formal or technical term for an engine or motor, often used in mechanical or engineering contexts. In everyday speech, エンジン or モーター are more common.
This engine runs on diesel fuel.
I read an engineering book to learn how engines work.
発動機 is a formal, technical term covering both motors and engines, whereas モーター is the everyday word usually meaning an electric motor.
Standard modern kanji spelling.
Out-dated kanji form using 發 instead of 発.
Compound of 発動 (hatsudō, 'starting up, operation') + 機 (ki, 'machine'). The term was coined in the Meiji era to translate Western concepts of engines and motors.