Pitch is the perceived frequency of sounds that allows them to be ordered on a scale from low to high. It is a major auditory attribute of musical sound, along with timbre, loudness and duration.
Frequency is a precise scientific unit of measurement measured in cycles per second (Hertz). Pitch is closely related to it but is not exactly the same because any sound produced by a musical instrument or other source consists of a set of frequencies rather than a single frequency. For most musical instruments, the pitch of a note will be its fundamental frequency, which is both the lowest and strongest frequency produced, with higher overtones occurring more quietly at multiples of the fundamental frequency.
Concert pitch is the pitch to which a group of musical instruments are tuned to for a performance. Today it is usually set at 440Hz for the A above middle C, although it has varied widely over musical history.
Not all musical instruments, namely some percussion instruments, produce notes with a clear pitch. However, is still possible for one of two sounds with indefinite pitch to be clearly higher or lower than the other.
Some musicians have absolute pitch or perfect pitch, which is the ability to know what note is being played, even without comparing it to another note. Having perfect pitch does not necessarily make someone a good musician, although it can be very useful.