Definition of concretion

i. A hard, compact mass or aggregate of mineral matter, normally subspherical, but commonly oblate, disk-shaped, or irregular with odd or fantastic outlines; formed by precipitation from aqueous solution about a nucleus or center, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock, and usually of a composition widely different from that of the rock in which it is found and from which it is rather sharply separated. It represents a concentration of some minor constituent of the enclosing rock or of cementing material, such as silica (chert), calcite, dolomite, iron oxide, pyrite, or gypsum, and it ranges in size from a small pelletlike object to a great spheroidal body as much as 3 m in diameter. Most concretions were formed during diagenesis, and many (esp. in limestone and shale) shortly after sediment deposition.

Compare with: nodule

ii. A collective term applied loosely to various primary and secondary mineral segregations of diverse origin, including irregular nodules, spherulites, crystalline aggregates, geodes, septaria, and related bodies. Not recommended usage.

Refs.:
AGI
https://en.wikipedia.org/wiki/Concretion