A organic chemical substance, with the structural formula R-COOH.
They often are volatile and some are responsible for foul odors.
Some can be quite corrosive and toxic such as Ethanoic/Acetic acid, Formic/Methanoic.
When in long chains they are known as fatty acids.
Vinegar is a diluted form of the carboxylic acid ethanoic acid traditional produced by fermentation of alcoholic beverages such as beer or cider.