In general, isolation is when someone or something is set apart or separated from other persons or things. In medical contexts, isolation specifically means “the complete separation from others of a person suffering from contagious or infectious disease.”
according to the CDC, the practice of isolation entails:
… the separation of a person or group of people known or reasonably believed to be infected with a communicable disease and potentially infectious from those who are not infected to prevent spread of the communicable disease. Isolation for public health purposes may be voluntary or compelled by federal, state, or local public health order.
The takeaway: isolation happens when a person is infected with a communicable disease, and is separated from people who are healthy. This also helps stop the spread of the disease.
Voluntary isolation is sometimes called self-isolation, although everyday people using the latter term may not mean they are actually infected.
First recorded around 1825–35, isolation ultimately comes from the same root as insulation: the Latin insulātus, “made into an island,” based on insula, “island.” Isolated is recorded around 1755–65.