Simple Definition:

An antacid is a type of medication that helps to neutralize [cancel the effects of] stomach acid [a sour liquid in the stomach that helps us digest food, and can cause hurtburn]. Antacids are commonly used to relieve symptoms of heartburn or indigestion. Examples of antacids include Tums, Rolaids, and Maalox.

Very Simple Definition:

An antacid is a medicine that helps with stomach problems. It can make your stomach feel better when it is hurting. Some antacids are Tums, Rolaids, and Maalox.