An octive is a musical inteval. It's rather hard for me to break it down scientificaly...It's like asking a math guy what is "three".

However, for the technical minded about you here's the definition of an octive from the Music Cognition Handbook from Ohio State University:

A musical term denoting an interval of pitch between two pitches whose frequencies are related in the approximate ratio of 2:1. Since exact octaves are frequently not always precisely related by a 2:1 frequency ratio, psychoacousticians have also defined the octave as the pitch interval between two tones such that one tone is perceived as duplicating the basic musical import of the other tone at the nearest possible higher or lower pitch. (American National Standards Institute, Psychoacoustical Terminology, S3.20, 1973; p.34). See also cent, semitone.