Something that, or someone who, improves something.
One who improves his or her performance, one who shows improvement (of individuals or groups).
A substance added to cause improvement (especially to a foodstuff).
A pad worn by women to make the dress hang properly.
Something that improves one's mood.
A drink, especially an alcoholic one, taken as a stimulant or sometimes as a hangover cure; a restorative, tonic or bracer.