Isolation is being separated from other people and your environment. Sometimes this occurs through decisions we make ourselves, or because of circumstances. Social isolation is an absence of social relationships. It is different from the state of being alone. Social isolation is can be measured in of the number of contacts that people have.
Loneliness and social isolation are different but are related Social isolation can lead to loneliness and loneliness can lead to social isolation. Both may also occur at the same time.