Believe it or not, vomiting isn't always vomiting and it's something that vets really do have to worry about. Let me give you a quick lesson so you can help your vet determine what is wrong with your pet.
Producing fluid or food from the mouth generally falls into two broad categories: vomiting and regurgitation. The distinction is important because each can point to different disease processes. Few clients understand this difference (which is what I'm hoping to correct) so most people simply think that their pet vomited. In a nutshell, vomiting is active and regurgitation is passive. But let's look at the two in a little more detail.
Regurgitation normally involves stomach contents coming up and out through the mouth without active muscle contractions. If the esophagus is enlarged and dilated (such as in megaesophagus), food can come back up because it sits in the esophagus rather than making it to the stomach. It can also happen if the upper sphincter in the stomach (the cardiac sphincter) is loose, not sealing food in the stomach. When regurgitation happens the pet usually just stands there, opens its mouth, and fluid or food comes out. Because there are no muscle contractions it just sort of falls out and there is no force behind it.
When a pet vomits, the esophagus and cardiac sphincter are functioning normally, pushing and keeping food into the stomach. When the nausea centers are triggered the stomach and abdominal muscles contract, resulting in the contents being expelled from the mouth. The difference is that there is force behind vomiting. You can see the abdomen contracting and the vomit may be "projectile", covering some distance. It's not uncommon to have multiple episodes of vomiting close together, which is much less common with regurgitation. You can often see a "lead-up" to the actual vomit where the pet feels the nausea and contractions then gets into the position to expel. Regurgitation more typically involves a sudden "splort" (my word) without any real warning.
We vets do try to differentiate between the two because the things that cause regurgitation are usually different than the causes of vomiting. It's kind of like trying to tell the difference between "dry" and "wet" coughs. So the next time your pet seems to vomit, try and pay close attention so you can give more information to your vet. It may actually make a difference with your pet's diagnosis.