Rumination is the chief component of rumination disorder, a rare eating disorder in infants and young children. Rumination in eating refers to regurgitating food and chewing it again, possibly multiple times. Whereas animals such as cows (classified as ruminants) do so by design, humans who ruminate are engaging in behavior considered abnormal.
The more common usage of the word "rumination" is a repetitive way of thinking, or mulling over a thought many times. It's a good metaphor for rumination in eating: Just as we sometimes process thoughts repeatedly, those with rumination disorder process food repeatedly.

