An antacid is a chemical that is swallowed in order to buffer gastric acid, raising the pH to reduce acid damage and relieve heartburn. Some also adsorb bile salts. Common types are:

Aluminum hydroxide and magnesium hydroxide (Maalox®, Mylanta®)
Aluminum carbonate gel (Basajel®)
Aluminum hydroxide (Amphojel®, AlternaGEL®)
Calcium carbonate (Tums®, Titralac®, Calcium Rich Rolaids®)
Magnesium hydroxide (Phillips’® Milk of Magnesia)
Sodium bicarbonate (Bicarbonate of soda)