Noun
1. (Geology), a small rock or mineral cluster. (wikipedia.org)
2. (Medicine), a small aggregation of cells. (wikipedia.org)
3. A rounded mass or irregular shape; a little knot or lump. (wiktionary.org)
4. (Minerology) A small rounded lump of mineral substance (usually harder than the surrounding rock or sediment). (wordnetweb.princeton.edu)
5. A small lump, swelling or collection of tissue. (biology-online.org)
Word origin: From Latin nodulus “small knot,” diminutive of nodus “knot”