An interval or tuning is said to be xenharmonic if it sounds strange or alien. The concept of something being xenharmonic is relative and also cultural. However, since most traditional music (indeed, most music, even atonal music) centers around tunings that provide good matches to natural harmonics, tunings and intervals that poorly match the more basic intervals of the harmonic series are likely to sound xenharmonic to almost anyone.
Certain intervals in 31 Tone Equal Temperament may sound xenharmonic to an unfamiliar ear, but 31-ET is arguably one of the least xenharmonic equally tempered microtonal tunings. The most xenharmonic interval in 31-ET is probably the semi-diminished fourth, and the fact that this interval is not particularly dissonant contributes to 31-ET as a whole not being particularly xenharmonic.
A tuning is generally considered to be xenharmonic if it lacks the familiar intervals associated with diatonic music, but contains other intervals roughly similar in size. For example, if a tuning lacks a perfect fourth or fifth, a major or minor third, or a whole tone, but matches intervals in between these intervals in width.
Tunings with xenharmonic properties that have seen some use: