Use the noun reflux to talk about a medical condition that gives you an acidic taste in your mouth and a burning feeling in your chest.
If a patient suffers from reflux, his stomach and esophagus aren't working quite right — they're allowing acid to flow backwards, which results in discomfort and nausea. There are various ailments that can be defined as reflux, including indigestion and heartburn, as well as conditions that affect babies who have trouble swallowing. Another kind of reflux is the tide's ebb, or outward flow. This is the oldest meaning of the word, which comes from the Latin fluxus, "a flowing."