WordNet sense 1 (confinement of a prisoner in isolation from other prisoners):

solitary confinement

WordNet sense 2 (one who lives in solitude):

hermit, recluse, troglodyte, solitudinarian

WordNet sense 3 (of plants and animals; not growing or living in groups or colonies):

nonsocial, nongregarious

WordNet sense 4 ((of animals) not gregarious):

ungregarious

WordNet sense 5 (lacking companions or companionship):

alone, lone, lonely

WordNet sense 6 (not seeking or given to association; being or living without companions):

unsocial

WordNet sense 7 (being the only one; single and isolated from others):

lonesome, only, sole

WordNet sense 8 (existing alone or consisting of one entity or part or aspect or individual):

single

WordNet sense 9 (remote or secluded):

unfrequented

WordNet sense 10 (capable of being reached only with great difficulty or not at all):

inaccessible, unaccessible