Medical Definition of cold sore
: a vesicular lesion that typically occurs in or around the mouth, that initially causes pain, burning, or itching before bursting and crusting over, and that is caused by a herpes simplex virus which remains dormant in the body and may be reactivated by a variety of factors (as stress, fever, or sunburn)—called also fever blister; compare canker sore
Seen and Heard
What made you want to look up cold sore? Please tell us where you read or heard it (including the quote, if possible).