loneliness

Short definition from Wiktionary

Noun

1. 1
   A feeling of depression resulting from being alone or from having no companions.
2. 2
   The condition or state of being alone or having no companions.
3. 3
   The state of being unfrequented or devoid of human activity (of a place or time).
4. 4
   A desire to be alone; disposition to solitude.