A physical core (also processing units) or just simply a core is well-partitioned piece of logic capable of independently performing all functions of a processor (i.e., central processing unit in the case of a general-purpose microprocessor). A single physical core may correspond to one or more logical cores.
In its simplest form, such as the case with single-core microprocessors, a core is often synonymous with a central processing unit. In those cases, the entire microprocessor itself is sometimes erroneously referred to as a CPU as well. With the proliferation of commercial multi-core microprocessors since the late 2000s, the term "core" has been used to more concisely describe the individual independent processing units within the microprocessor.
It should be noted that not all cores are created equal - even within the same microprocessor. The exact capabilities of each individual core is highly dependent on the design and architecture of the system. For example, the Intel's Core i7-7820X has eight identical cores on a single integrated circuit, each capable of operating on equally heavy workloads simultaneously. Unlike Intel's, Samsung's eight-core Exynos 7420 makes use of a big.LITTLE architecture whereby half of the cores are powerful (big) and the other half are power-efficient and far less powerful (little) cores. Depending on the workloads the microprocessor may shift workloads between the cores in order to deliver higher performance or high power efficiency.
The term "multi-core" refers to microprocessors that incorporate multiple physical cores on a single chip. For example a "dual-core microprocessor" refers to a microprocessor that incorporates two cores while a "dodeca-core microprocessor" is another such processor that incorporates twelve cores.