The diesis is the smallest interval in 31 equal temperament. It can be seen as the building block of all the other intervals. The diesis is narrower than an equally-tempered quarter tone.
When used harmonically, the diesis is extremely dissonant. The notes are far enough apart that they usually sound like distinct notes rather than just an out-of-tune attempt at unison, but this depends both on range, timbre of the instruments, harmonic context, and a person's ear. Someone not used to hearing intervals this narrow may perceive the diesis as simply being an out-of-tune unison.
In more general music theory, the term diesis can refer to a number of different musical intervals. The diesis of 31-ET is the closest match to the undecimal diesis, which corresponds to the natural harmonic ratio of 45:44. This interval is the difference in just intonation between the major third and the undecimal neutral third.