An antacid is a medicine you take when your stomach burns. That burning often comes after you eat too much, eat very oily food, or drink too much alcohol: extra acid in the stomach rises and hurts. The antacid works against that acid and calms the burn. People buy it at a pharmacy; it can be a chewable tablet, a liquid, or a fizzy drink. It does not fix every stomach problem — it mainly helps with that acid burn, which doctors call heartburn or acid indigestion. It is an everyday, slightly medical word. You say it at the chemist, not in a story-book. It can also be an adjective: antacid tablets.

制酸劑（緩解胃酸、胃灼熱）。口語常籠統叫「胃藥」，但胃藥還包括止吐、胃散等，制酸劑才是針對胃酸的那種。