A microchip that combines multiple core functions on one chip, enough to form most of a system.
In a phone, the SoC is a single chip that typically combines a CPU (a processor, the brains), GPU (graphics, to drive the display), and a modem (to communicate with the cellular mobile network), as well as other key functions.
Because the application processor (CPU) is the most critical part of an SoC, SoC chips are sometimes just called "processor" chips, and the terms are used interchangeably. But SoC is technically more accurate because an SoC includes much more than just the "processor" part.
Examples of SoCs include Qualcomm's Snapdragon line of chips, MediaTek's Helio and Dimensity lines of chips, and Samsung's Exynos.