adjective
(a) (of person) lively, energetic; although he is over eighty he is still very active
active movement = movement made by a patient using his own willpower and muscles
(b) (of disease) which affects a patient, which is not dormant; after two years of active rheumatoid disease
(c) which acts, does something
active ingredient = main medicinal ingredient of an ointment or lotion (as opposed to the base)
active principle = main medicinal ingredient of a drug which makes it have the required effect on a patient