What is it called when undisturbed sedimentary rock layers occur in horizontal layers?

When undisturbed sedimentary rock layers occur in horizontal layers, it is referred to as the principle of original horizontality. This geological principle states that sedimentary rocks are initially deposited in horizontal layers due to the influence of gravity. If the layers are found tilted or folded, it indicates that geological processes, such as tectonic activity, have occurred after their formation.