Medical Definition of active
1: capable of acting or reacting especially in some specific way <an active enzyme> <active nitrogen>
2: tending to progress or to cause degeneration <active tuberculosis>
3: exhibiting optical activity
4: requiring the expenditure of energy <active calcium ion uptake>
5: producing active immunity <active immunization>
Seen and Heard
What made you want to look up active? Please tell us where you read or heard it (including the quote, if possible).