According to WebMD, cat vomiting up clear liquid indicates mild intestinal distress, especially when caused by fibrous material the cat has swallowed, such as long fur, grass or string; cats cannot digest such material. Often such matter passes straight through the cat intact, but if it gets stuck in the stomach, it may get hacked back up through the mouth.
A cat vomiting up hair or grass is not a serious medical concern. Cats, especially long-haired breeds, commonly vomit up hairballs, according to Vet Info. Although messy, this can easily be prevented by frequently brushing the cat. If a cat is hacking up clear liquid without any accompanying solid matter present in the vomit, that may indicate that the material causing the vomiting is stuck inside the cat's gastrointestinal tract. If that is the case, the cat should be taken to a veterinarian immediately to remove the obstruction, according to the ASPCA. Another serious condition that includes the symptom of clear vomit is infectious enteritis, or inflammation of the small intestine. According to Medline Plus, this is a form of food poisoning and also requires a visit to the vet. Two rarer conditions that can cause this symptom are cancer and thyroid disease. Both issues may be difficult to distinguish from food poisoning when they first present gastrointestinal symptoms.