This should be on the Wiki or Open Music Theory sites. (The names of non-harmonic tones isn't consistent.)
A complete neighbor would something like C-D-C (upper neighbor) or C-B-C (lower neighbor). The complete neighbor occurs between two occurrences of the same chord-tone. An incomplete neighbor would be something like G-B-C where the G is consonant. Its not a passing tone (E-D-C for example) so it's called an incomplete neighbor.