A dose of powder compressed into a solid pill, often with binders and a coating.
A tablet is made by compressing powdered ingredients into a solid pill, usually with binders, fillers, and sometimes a coating to hold it together and control how it breaks down. Tablets can pack a large dose into a small pill and tend to cost less to produce, which is why bulk minerals like calcium and magnesium are often sold this way.
The trade-off is more non-active ingredients than a capsule and, occasionally, a hard-pressed tablet that dissolves slowly.