Mudstone

A mudstone is a siliciclastic sedimentary rock which has a mixture of silt- and clay-sized particles (at least 1/3 of each). So, a mudstone is a type of mudrock with a more varied grain size than either siltstones or claystones.
Alternatively, some sources say it is a synonym for mudrock.
Claystone.
A claystone is a mudrock which does not split easily.
In order for a rock to be a claystone, it must be up to half (50%) clay, whose particles measure less than 1/256th of a millimeter.