Medical Definition of free radical
: an especially reactive atom or group of atoms that has one or more unpaired electrons; especially : one that is produced in the body by natural biological processes or introduced from outside (as in tobacco smoke, toxins, or pollutants) and that can damage cells, proteins, and DNA by altering their chemical structure
Seen and Heard
What made you want to look up free radical? Please tell us where you read or heard it (including the quote, if possible).