A PCB is called Multilayer PCB, if it has 3 or more layers(sheets) of copper foil, insulated from each other(by lamination or coating) and joined together to form a single piece.
A Multilayer PCB has an Inner Layer Core present at the center of all the layers and coated with a Copper layer from both sides, while layers of prepreg(woven glass cloth with epoxy resin) are used for insulation.
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In Multilayer PCB, outer layers(top & bottom) are single-sided PCBs, while the inner layers are double-sided PCBs.