Loneliness
Loneliness can affect anyone, at any time in their life. It is more than just being alone, although being isolated and alone can increase feelings of loneliness. It is subjective and what feels lonely for one person might be perfectly ok with another.
Social isolation
Social Isolation can lead to feelings of loneliness but is in itself the state of being distanced or isolated from other people. Some people like or have chosen to be isolated and it is possible to be isolated and still have many great friendships which prevent you from becoming lonely.