"Tart cherries" refer to a specific type of cherries that have a sour or tangy taste compared to other varieties of cherries, which are usually sweeter. Full definition
Volunteers were instructed to drink 8 ounces of tart cherry juice concentrate in the morning and 1 to 2 hours before bed for two weeks. (healthyeating.sfgate.com)
I keep hearing a billion and one benefits of tart cherry juice as well — I need to get on that! (peasandcrayons.com)
I have a bag of dried tart cherries in my pantry right now. (cookincanuck.com)