Tablet can mean several things:
Pharmacologically, a tablet is a medicinal or other active substance mixed with binder powders and pressed into a tablet form.
Most tablets are circular, or disk-shaped, but recent decades have seen many that are oblong or various-shaped. When Tylenol (acetaminophen) capsules were laced with cyanide, many people stopped buying capsules because they are easy to contaminate, in favor of tablets, which are not. Some makers of OTC (over-the-counter) drugs responded by starting to make what they termed "caplets", which were actually just tablets made in the shape of a capsule.
In the tablet-pressing process, it is important that all ingredients be dry, powdered, and of uniform grain size as much as possible. Mixed grain sizes tend to separate out due to operational vibrations, resulting in inconsistent tableting, while any moisture in the system will tend to clog the tableting pathways.
Some substances may be tableted as pure substances, but this is usually not the case. Normally, an inactive ingredient termed a binder is added to help hold the tablet together and give it strength. A wide variety of binders may be used, some common ones including lactose powder, sucrose powder, tapioca starch (cassava flour) and microcrystalline cellulose.
Often, an ingredient is also needed to act as an excipient. This is an ingredient that dissolves readily in water to help the tablet disperse once swallowed. Some binders, such as sucrose, are also excellent excipients.
Small amounts of lubricants are usually added, as well. The most common of these are stearic acid (stearin) and magnesium stearate. These help the tablets, once pressed, to be more easily ejected out of the die.
Many tablets today are coated after being pressed. Some coatings are just to provide color or a smooth finish, or to facilitate printing on the tablet (although characters and symbols are easy to emboss into the tablets using special die sets). Coatings are often sugar-based (the old "sugar-coating"!) to make pill-taking less unpleasant. Some tablets, however, have a special coating termed an enteric coating, which is resistant to stomach acid and takes time to wear away. The purpose of this coating is to prevent dissolution of the tablet in the stomach, where the stomach acid may neutralize the active ingredient, or where the time of passage may compromise its effectiveness, in favor of dissolution in the small intestine, where the active principle is better absorbed.
Tablet presses, the machines that make the tablets, range from small, inexpensive bench-top models that make one tablet at a time, no more than a few thousand an hour, and with only around a half-ton pressure, to large, computerized, industrial models that can make hundreds of thousands of tablets an hour with much greater pressure. Some tablet presses can make extremely large tablets, such as some of the toilet cleaning and deodorizing products.