While many often use the terms "regurgitation" and "vomit" interchangeably, the difference is important to identify so that you may more accurately determine the source and severity of a potential medical problem.
First, it helps to understand the basic anatomy and function of a snake's digestive system. When a feeder is ingested, it passes through the snake's esophagus, which can extend as far as half of the snake's body. From the esophagus, the feeder then moves into the stomach, where most of the digestive process takes place.
After the feeder is broken down in the stomach, it is passed into the small intestine, where nutrients from the meal will be absorbed into the body. The liver and pancreas also aid in this process by excreting enzymes into specific regions of the small intestine. Once the feeder passes through the small intestine, it then moves into the large intestine, where the formation of feces begins and is stored. From there, excrement is passed into the cloacal chamber and divided to receive uric products and feces, and then finally expelled from the cloaca.
Now that we have a general understanding of a snake's digestive system, we can better grasp what regurgitation and vomit are from a medical standpoint. Regurgitation is an undigested feeder that has remained in the upper gastrointestinal tract (up to the first portion of the small intestine) and will typically occur within 4-5 days after feeding. Vomit on the other hand is a partially digested feeder that has reached the lower gastrointestinal tract and can occur anywhere from 5 days to a month after feeding. WARNING: GRAPHIC CONTENT Pictures featuring regurgitated and partially digested mice are below the line
Regurgitation (upper GI tract) Vomit (lower GI tract)
The difference is significant because vomit is typically a more serious matter, as it means the feeder has had enough time to enter the later stages of decomposition inside of the snake, which can cause a fatal internal bacterial infection. While both regurgitation and vomit are concerning, regurgitation is not always grounds for a vet visit -- vomit, however, is more serious, and a medical checkup is strongly suggested. Internal bacterial infections as a result of partially digested feeders rotting in the stomach can kill a snake within a week if left untreated. Other signs of internal bacterial infection include lethargy, drastic weight loss, and lack of appetite. If you notice any of these signs in your snake, especially after vomit, seek a vet immediately. Regarding the visual differences between vomit and regurgitation, the two are easy to tell apart if you know what to look for. You will be able to see most if not all of the feeder intact with a regurgitation. With vomit, however, it will look partially digested and usually attached to what will appear similar to fecal matter (but is not, by definition) since it has been partially digested in the stomach. See the images below for comparison.
As for the cause of regurgitation vs. vomit, both are usually a result of low temperatures within the enclosure. The difference is that when vomit occurs as a result of low temperatures, it typically means the temperatures were high enough to begin digesting, but then dropped too low to continue to digest. That said, there are various other factors that can contribute to either vomit or a regurgitation, including parasites, viruses, obstructions, etc. While these are less likely the source of the problem, any additional signs of illness should warrant professional veterinary assistance as soon as possible. Regardless of the cause however, it is crucial to always wait two weeks before attempting another feeding if your snake regurgitates and up to three weeks if your snake vomits. While the difference in terminology may seem insignificant to some, we should get into the habit of using the terms "regurgitation" and "vomit" correctly so that we can more accurately gauge the severity of a potential medical problem. As loyal caretakers for these wonderful animals, it is our responsibility to assure their health and happiness to the highest degree; and the more you know about them, the better off they will be!