A nodule is a small, solid lump or bump that forms in or on something. Doctors might find a nodule on someone's thyroid gland during a checkup, which is usually harmless but needs to be examined. Miners discover nodules of precious metals embedded in rock. Marine biologists study manganese nodules, potato-sized lumps of minerals that lie scattered across the deep ocean floor.

The word describes something rounded and compact, like a little knot or knob. Root nodules are small bumps that form on the roots of plants like peas and clover. These nodules contain helpful bacteria that pull nitrogen from the air and make it available to the plant, creating a partnership that helps both organisms thrive.

In geology, a nodule often refers to a rounded mineral formation that's different from the rock surrounding it. You might crack open a plain-looking rock and find a beautiful crystal nodule inside, compact, rounded, and distinct from what surrounds it.