Gruel is a food preparation consisting of some type of cereal — oat, wheat or rye flour, or rice — boiled in water or milk. It is a thinner version of porridge that may be more often drunk than eaten and need not even be cooked. Historically, gruel — often made from millet, hemp or barley, or in hard times, of chestnut flour and even the less tannic acorns of some oaks — has been a staple of the human diet, especially for peasants.
The importance of gruel as a form of sustenance is especially noted for invalids and for recently-weaned children. Hot malted milk is a form of gruel, although manufacturers like Ovaltine and Horlicks avoid calling it gruel, due to the negative associations attached to the word through novels like Charles Dickens’ Oliver Twist.