All other intervals have at least two names, based on enharmonic spellings. But the “tritone” is unique. There is only one interval “between” the perfect 4th and the perfect 5th. No matter what harmonic spelling is used, the name for this interval remains the same: tritone.
Note to more advanced students: the tritone is also labeled according to spelling, so there are at least two common terms that are used to describe the way it is notated—augmented 4th and diminished 5th. However, both of these are tritones, so these extra terms are not needed to describe what we hear. In addition, we can only say that an augmented 4th and diminished 5th are exactly the same when referring to equal-temperament, which is the tuning system used for all modern pianos and the default tuning used for digital keyboards.

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