A core, or CPU core, is a processing unit of the CPU (central processing unit) that is responsible for executing programs and multiple other actions on a computer. It is the "brain" of a CPU and receives instructions and performs calculations or operations to satisfy those instructions. Each core can perform operations separately from the others, and multiple cores may also work together to perform parallel operations on a shared set of data in the CPUs memory cache. A CPU can have multiple cores, and processors with two cores are called dual-core processors, with four cores, quad-core, six cores, hexa-core, and with eight cores, octa-core. As of 2023, most consumer CPUs feature between two and twelve cores, while workstation and server CPUs may feature as many as 48 cores.

The number of cores in a CPU has a direct effect on the performance of a computer, but not in any simple linear way. The more cores a CPU has, the more efficient it is, as each core can work on a different task. CPUs with more cores are better suited for tasks that require more processing power, such as video editing, graphic design, 3D rendering, and sound editing. However, for standard computing, a CPU with at least two cores is sufficient.

In summary, a core is a small CPU or processor built into a big CPU or CPU socket that can independently perform or process all computational tasks. The number of cores in a CPU affects the performance of a computer, and CPUs with more cores are better suited for tasks that require more processing power.