Perfect pitch, also called absolute pitch, is the ability to hear and identify a given note without any musical or tonal support. In the case of a singer, having perfect intonation, the ability to sing without making significant pitch errors, is not the same as having perfect pitch.
People with perfect pitch may be able to identify and name individual pitches played on various instruments, name the key of a given piece of music, identify and name all the tones of a given chord, and/or sing a given pitch without external reference. Compare with relative pitch.
Absolute pitch is really an innate form of memory: possessor of it retains in their mind (consciously or unconsciously) the pitch of some instrument to which they have been accustomed and instinctively relates to that pitch every sound heard. Many good musicians possess this faculty; as many other do not: its possession or non-possession is, then, not in itself a proof of either high or low musical capability.
The possession of absolute pitch can be extremely useful , but may sometimes cause problems, as, for instance, when a singer with this faculty is called upon to sight-read accompanied by an instrument tuned to what is to them 'the wrong pitch', necessitating on the singer's part a conscious transposition of the vocal line that is before them in the notation.