The term SoC stands for system-on-chip. Unlike the x86 architecture, where the processor and the various peripheral controllers are on different chips (North-bridge, south-bridge, etc.), SoCs integrated these onto a single chip.
Wikipedia 1 defines it as thus:
System-on-a-chip or system on chip (SoC or SOC) refers to integrating all components of a computer or other electronic system into a single integrated circuit (chip). It may contain digital, analog, mixed-signal, and often radio-frequency functions – all on a single chip substrate. A typical application is in the area of embedded systems.