In music, the note one semitone above B is C. On a piano, B is the white key immediately to the left of C, so moving upward by one semitone reaches C without passing through a black key.
A semitone, also called a half step, is the smallest interval in standard Western equal-tempered music. Most neighboring piano keys are one semitone apart, including B–C and E–F. These pairs are commonly remembered because they have no black key between them. By contrast, B to B-sharp is written as an enharmonic spelling of the same pitch as C, but B-sharp is not the usual canonical answer to this question.
The letter names repeat in cycles from A through G, so C is both a note name and the beginning of the familiar C-major scale: C, D, E, F, G, A, B. In equal temperament, the frequency of C is approximately 261.63 Hz when it is middle C, while the B immediately below it is approximately 246.94 Hz. The precise frequency changes with octave and tuning system, but the one-semitone relationship remains.