Solitude

The noun solitude (the state of being alone, remoteness from society) came to English in the mid-14th century from 14th century Old French solitude (loneliness). These words are from Latin solitudinem (loneliness, a being alone; a lonely place, desert, wilderness) and solus (alone).
 
The Online Etymological Dictionary, referring to the Oxford English Dictionary (OED), states that the word solitude was “not in common use in English until the 17th century.”
 
Interestingly, the 17th century was also the time when the words individual and individualism had come to “indicate separateness.” An individual is “one that exists as a distinct entity” in relation to a community or society. An individual is “a person unique from other people and possessing their own needs or goals, rights and responsibilities” (Wikipedia).
 
In brief, the noun solitude means the state of being alone. Solitary is an adjective meaning alone or without companions.
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/
https://en.wikipedia.org/wiki/Individual