In Western music, the expression "middle C" refers to the note "C" (or "Do" in fixed-Do solfege) located exactly between the two staves of the grand staff, quoted as C4 in scientific pitch notation (also known as note-octave notation). It also tends to be to the middle of a keyboard, and it is near the top of the male vocal range and the bottom of the female vocal range.
Although C4 is commonly known by the expression "middle C", the expression is somewhat keyboard-specific: players of some instruments may refer to the note by another name, and may use "middle C" to refer to a different note. For example, that note (C4) would be "low C" to the player of a Western concert flute (as it is in the lowest register of that instrument — see playing range), while C5 would be middle C. Nevertheless, the expression "middle C" is generally clear across instruments and clefs.
When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of the middle C note is approximately 261.626 Hz.