A small, knoblike outgrowth found on the roots of many legumes, such as alfalfa, beans, and peas. Nodules grow after the roots have been infected with nitrogen-fixing bacteria of the genus Rhizobium. See more at legume.
A small, rounded lump of a mineral or mixture of minerals that is distinct from and usually harder than the surrounding rock or sediment. Nodules often form by replacement of a small part of the rocks in which they form.