Dental decay is the same as tooth decay. It is also known as dental caries. It results in a cavity in the tooth and if untreated can spread from the enamel (the hard outer covering of the tooth) into the softer dentine inside.
The decay can progress more rapidly in the softer dentine and will eventually reach the dental pulp (nerve and blood vessels) in the centre of the tooth and cause this to become infected and inflamed. Once the pulp is infected it may die and bacteria can breed in the empty pulp chamber pushing beyond the end of the tooth root to form a dental abscess.