It is the second semitone in the French solfège and is known there as do dièse.
In equal temperament it is also enharmonic with B (B-double sharp/Hisis).
When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of C♯4 (the C♯ above middle C) is about 277.183 Hz.
[1] See pitch (music) for a discussion of historical variations in frequency.
This music theory article is a stub.