unique

Concise Oxford English Dictionary © 2008 Oxford University Press:
unique/juˈniːk/

▶adjective

• 1 being the only one of its kind; unlike anything else.

  ■ (unique to) belonging or connected to (one particular person, place, or thing).

• 2 special or unusual.

– derivatives
uniquely adverb,
uniqueness noun.

– origin C17: from Fr., from L. unicus, from unus ‘one’.

usage: Strictly speaking, since the core meaning of unique is ‘being the only one of its kind’, it is logically impossible to modify it by adverbs such as really or quite. However, unique has a less precise sense in addition to its main meaning: ‘special or unusual’ (a really unique opportunity). Here, unique does not relate to an absolute concept, and so the use of really and similar adverbs is acceptable.