Gravy is really meat essence, usually diluted by washing and stirring round the cooking vessel with a little water, as in the making of gravy to serve with roast meat. Gravy may or may not be thickened according to custom in the accompaniments for different kinds of meat.
In the making of gravy for roast meat it is an excellent modem practice to use the water in which green vegetables have been cooked, the only other addition that is permissible is of a good meat or yeast extract.