Ditave is an alternative name for the interval Octave, which was proposed to neutralize the terminology against the predominance of 7-tone scales. The name is derived from the numeral prefix δι- di- (Greek for two) in analogy to "Tritave" (3/1). A brief but complementary description about it is here.
Diapason is another term also sometimes applied to the 2/1 interval (octave). It is also of Greek origin, but not related to the number two; instead it is formed from διά dia + πασων 'pason, meaning something like "through all the notes".