In medical language, a bolus is a single concentrated dose of a substance given at one time, often as a large pill or lump of medicine. In eighteenth-century usage, it commonly referred to a soft, rounded medicinal preparation taken by mouth, sometimes as a purgative or treatment for illness. In poetry, the word can suggest bodily discomfort, disease, or the unglamorous realities of medical care.

The word comes from Greek bolos, meaning a lump, clod, or mass. In modern medicine, a bolus may refer more broadly to a single dose administered quickly, including an injection or intravenous medication. In digestion, a “food bolus” is the chewed mass of food formed in the mouth before swallowing; this is a different but related use of the term. In older poetry and prose, a bolus usually means an orally taken medicine rather than a food mass. Related terms include dose, pill, draught, potion, and purge.