Sole; being the only one of a type.
Situated by itself or by oneself, with no neighbours.
Solitary; having no companion.
Isolated or lonely; lacking companionship.
(of an adjective) Describing a quality that cannot normally be varied because it is extreme (e.g. freezing), absolute (e.g. dead), or classification (e.g. nuclear).
That cannot be graded.
Any adjective that is not gradable.