The term 'The Loneliest Number' is a phrase that has been used in popular culture to describe the number ONE. It is often used to describe a feeling of loneliness or isolation, and is often associated with the song 'ONE' by Three Dog Night. The crossword solutions for this term are ONE (3 letters) and ONEIS (5 letters). The phrase 'The Loneliest Number' is a metaphor for the number ONE, which can be seen as a symbol of loneliness and isolation.