A single-core processor is a microprocessor with a single core on a chip, running a single thread at any one time. The term became common after the emergence of multi-core processors (which have several independent processors on a single chip) to distinguish non-multi-core designs. For example, Intel released a Core 2 Solo and Core 2 Duo , and one would refer to it as the ‘single-core’ variant. Most microprocessors are multi-core and are single-core. The class of many-core processors follows one from multi-core, in a progression showing Increasing parallelism over time.
Processors remained single-core until it was impossible to achieve performance gains from the increased clock speed and transistor count allowed by Moore’s law (there were diminishing returns to increasing the depth of a pipeline , increasing CPU cache sizes, or adding execution units ). 
Increasing parallel trend
- Single-core – one processor on a die.
- Multi-core – a few processesors on a die, eg 2,4,8. As of 2016, most CPUs fall into this category.
- Many-core – a ‘large number’ of processors on a die, eg 10s, 100s, 1000’s. Some specialist ASICs / Accelerators and GPUs fall into this category.
- Jump up^ “architecting solutions of the manycore era” .