Any adjective that is not gradable.
That cannot be graded.
(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).
Something that is done, created, etc. only once, and often quickly, simply, or improvisationally.
Something unique and remarkable.
Occurring once, independent of any pattern; one-time.
Singular; unique; special; remarkable.