a. The process by which food is converted into substances that can be absorbed and assimilated by a living organism. In most animals it is accomplished in the digestive tract by the mechanical and enzymatic breakdown of foods into simpler chemical compounds.i•ges•tion. 1. the process in the alimentary canal by which food is broken up physically, as by the action of the teeth, and chemically, as by the action of enzymes, and converted into a substance suitable for absorption and assimilation into the body.