Delta-rational chord
Pseudo-JI is a name given by Tom Price to an acoustic property that (usually non-JI) chords which have dyads which are close to having simple integer relationships in frequency differences (rather than frequency ratios as in the case of JI). For example, the 13edo chord 0\13-3\13-8\13-10\13 (0¢-185¢-738¢-923¢) satisfies the property because the dyad 8\13-10\13 in the chord has a frequency difference 0.994 times the frequency difference of the dyad 0\13-3\13. (In 0\13-3\13-8\13-924.159¢, the 3rd and 4th notes would have exactly the same frequency difference as the dyad 0\13-3\13.) The effect is thought to be caused by interference beating between the fundamentals and lower harmonics of the fundamental. The effect may be more or less pronounced depending on register, timbre, the complexity of the linear relationship, etc. For example, the pseudo-JI acoustic effect would be weaker in chords with very spaced-out voicing, as well as chords played in timbres with loud higher harmonics (because the higher harmonics would make the pseudo-JI relationships less obvious).
JI chords and isodifferential chords (chords of the form α : α + k1 : ... : α + kn for any positive number α and integers k1, ..., kn) are a special case of pseudo-JI, but in these chords all dyads in these chords are related in frequency space.
Definitions
Tom Price believes that a purely mathematical definition won't capture the set of chords that have the acoustic effect from interference. We will adopt the following mathematical definition for convenience: A chord C = α1:...:αn is mathematically pseudo-JI when the chord has two distinct dyads αk1:αk2 and αk3:αk4 such that (αk2 − αk1)/(αk4 − αk3) is rational.