Gastritis is a term used to describe irritation of the stomach, which causes symptoms such as vomiting and stomach pain.
There are many different causes of gastritis, some minor and some more serious. Gastritis usually resolves one to two days after treatment, however, some conditions (e.g. food allergies, kidney or liver disease) cause gastritis that comes and goes.
Contact your vet if your cat has been vomiting and you are concerned.