Eat, swallow, ingest, devour, consume mean to take food into the stomach through the mouth and throat.
Eat, the common and ordinary term, implies the process of chewing as well as of taking into the stomach and therefore distinguishes itself from swallow, which implies merely the passing from the mouth through the throat to the stomach.
Eat is often used, however, without any clear reference to chewing or swallowing and, especially in extended use, without implying anything but a slow, gradual process that is comparable to the biting or gnawing that precedes eating in that it wastes or wears away the substance.
In many idiomatic expressions the literal phrasing recalls the implications of the original meaning but nothing more.
Swallow basically implies the second part of the eating process. More often it is used of hurried eating without proper mastication of food.
In extended use it implies a seizing and taking in or a being seized and taken in (as by engulfment, engrossment, or suppression).
Ingest is a physiological term that implies a taking in through the mouth and throat into the stomach and is commonly opposed to egest.
Devour throws the emphasis on greediness; it suggests intense hunger or gluttony in man and voracity in a wild animal.
In extended use devour applies to something (as fire or disease) which destroys or wastes completely or to something which preys upon one as insistently as a beast or bird of prey.
Sometimes, however, it approaches swallow in its reference to something which engrosses the mind, but it heightens the implications of avidity and zest in taking in.
Consume (see also WASTE, MONOPOLIZE) usually means little more than eat and drink, for which it serves as a term including both or either. Very often, however, it adds to eat the implications of using up.