Multilayer
Multilayer are PCBs in form of multiple circuit layers stacked together.
Few already consider 2-layer circut boaders as multilayer, but generally speaking boards with at least 4 or more layers are considered as multilayer. 2-layer circuit boards are sometimes referred to as bi-layer. The upper limit for the amount of layers is wide open.
However, 4- and 6-layers dominate the multilayer area by taking about 90% of the market. Multilayer with up to 24 layers can be produced in our German factory. Up to 96 layer multilayer are rare, highly complicated, and expensive products.