A direct octave occurs when two voices move in the same direction, the upper voice leaping, and produce a vertical octave. This voice leading is normally considered a (soft) error, although composers did accept it under certain conditions, for instance by including a suspension in an inner voice at the moment of the direct octave.
See also direct fifth.
Select Bibliography
Demeyere, Ewald. Johann Sebastian Bach’s Art of Fugue — Performance Practice Based on German Eighteenth-Century Theory (Leuven: Leuven University Press, 2013).