When defining what is the TOP of your PCB, consequently the opposite side will be called BOTTOM. The bottom side of the PCB is usually the side without components and the side that touches the solder wave during assembly. That is why sometimes it is also called SOLDER side. However more often, PCB are populated on both sides and the assembly process does not require wave soldering. Hence BOTTOM side is a clearer definition. Read our 10 rules for better data.