In Israel, a year of military service for all citizens is compulsory––your only alternative to service is to go to jail. If something is compulsory, it is required or must be done.
This adjective is from Medieval Latin compulsorius "using force," from Latin compulsus, from compellere "to force." The Latin suffix –orius corresponds to English -ory "containing or involving." Other English words descended from the same Latin verb are compel and compulsive.