Antacid is a substance that reduces the level of acid in the stomach. a substance that gets rid of the burning feeling in your stomach when you have eaten too much, drunk too much alcohol etc. Any substance, such as sodium bicarbonate, magnesium hydroxide, or aluminum hydroxide, used to relieve the discomfort caused by indigestion, gastritis, and several forms of ulcers. Antacids counteract or neutralize gastric acidity for up to three hours after a single dose. Antacids should be taken when gastric acidity is most likely to be increasing namely, between one and three hours after each meal and at bedtime
an agent that counteracts or neutralizes acidity (especially in the stomach) acting to neutralize acid (especially in the stomach)