It is a note a half step above or below the original note (a sharp or flat).
The 7th note (leading note) is raised by a semitone in a harmonic minor scale.
It is half a note in pitch
Any note which has the word 'sharp' in it is always a semitone above the given note.
G#
semitone. B to C and E to F are semitones
flat
B
Lower each note by a semitone.
It lowers the note by one half-step/semitone.
It either raises or lowers a note a semitone. It cancels the effect of a sharp or flat.
In Music, a sharp is a symbol indicating the note is to be raised by one semitone. for example: A-sharp (A#) is the note one semitone (or one half-step) above A.
A semitone is Half a Note or the distance between one fret on a guitar e.g. C to C# or G to G#