Absolute adjective

From Teflpedia

An absolute adjective or ungradable adjective is an adjective that describes something that it logically binary, i.e. it is not gradable, and therefore is defective and lacks comparative and superlative forms and cannot be used with intensifiers.

For example, "She's dead".

Compare gradable adjective, also strong adjective.

In colloquial speech, "It was the most unique thing!" is generally accepted, but that would be proscribed from more formal registers.