碎屑岩是由于机械破碎的矿物和岩石碎屑，经过搬运、沉积、压实、胶结，最后形成的新岩石；成分除碎屑物外，还有含量较少的化学物质。

如果残余的碎屑是从沉积区外面的陆地上搬运过来的，叫做陆源碎屑或外碎屑，如果是在沉积区内形成的，叫做内碎屑，内碎屑很少见。

按照碎屑颗粒的大小，碎屑岩可分为：

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.