Vomiting is the forceful expulsion of undigested food or stomach contents. The active effort of vomiting will usually require your dog to visibly contract the belly. Vomiting is to be distinguished from regurgitation in which there is a backflow of food contents without any active effort on the part of your dog. With regurgitation your dog may cough, but will not heave his belly.  

Vomiting can be the result of a multitude of underlying conditions from a minor stomach upset, fear, or excitement to a true medical emergency such as an ingestion of toxic substance, acute pancreatitis, or even bloat. Vomiting can also be caused by many underlying diseases and condition such as ulcers, tumors, diabetes mellitus, or worms.