Wikipedia
In music theory, a leading-note (also subsemitone, and called the leading-tone in the US) is a note or pitch which resolves or "leads" to a note one semitone higher or lower, being a lower and upper leading-tone, respectively.
More narrowly, the leading tone is the seventh scale degree of the major scale, with a strong affinity for and leading melodically to the tonic . It is sung as ti in movable-do solfège. For example, in the C major scale (white keys on a piano, starting on C), the leading note is the note B; and the leading note chord uses the notes B, D, and F: a diminished triad. In music theory, the leading note triad is symbolized by the Roman numeral vii. By contrast, an upper leading-tone (; ), which leads down, may be found as the seventh of the dominant seventh chord, which leads to the third of the tonic chord (in C: F of a G7 chord leads to E of a CM chord). The upper leading-tone may also be found above the tonic, on D or C in C.