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I guess it's an augemented fourth because it contains a half step more than a perfect fourth; [from a source] but in a minor scale it isn't allowed: an augmented fourth is IV degree.

Gogehat
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    Interval naming has nothing to do with scales. It _can_ be convenient to think of interval names in terms of the Major scale, but interval names are only about the distance between note names. –  Mar 01 '18 at 02:10
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    Possible duplicate of [Are intervals like major 3rd, minor 3rd, and major 2nd all based on the scales, or are they based on how many semitones they have?](https://music.stackexchange.com/questions/61074/are-intervals-like-major-3rd-minor-3rd-and-major-2nd-all-based-on-the-scales) –  Mar 01 '18 at 02:32
  • [Another possible duplicate](https://music.stackexchange.com/questions/60771/defining-intervals) –  Mar 01 '18 at 02:36
  • @David Bowling - Would '*interval numbers* are about the distance between note names' be more accurate? – Tim Mar 01 '18 at 09:02
  • @Tim -- probably. Maybe 'interval names are are about distance and can be found by using the distance between note names and the distance between notes in semitones together in combination.' It seems difficult to capture an accurate description in a single sentence. –  Mar 01 '18 at 13:49
  • @DavidBowling - what I'm getting at is that say, C>E will always be a 3rd of some description, Bb> F always a 5th , as in the letter names give the interval *number*. The m/M, dim/aug part is sort of secondary, if that makes sense. – Tim Mar 01 '18 at 14:46
  • @Tim -- I understood your point, and that makes perfect sense. I came up a little bit short trying to describe interval naming in a pithy one-liner. –  Mar 01 '18 at 15:00

3 Answers3

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Any A to any D is some kind of 4th since the distance in letter names is that of a 4th. Since A to D is a perfect 4th, making the interval bigger by one semitone from A to D♯ makes this interval an augmented 4th.

For more information about naming intervals in general, please see one of the many questions on the site addressing this:

Dom
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Depends on which note is the lower. From D# to A, one has D#,E,F,G,A (with 5 letters) the interval is a diminished fifth. Going from A to D# one has A,B,C,D# and 4 letters so it's an augmented fourth.

ttw
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I've always referred to that interval as the Tritone and folks seem to know what I'm talking about. Augmented fourth or diminished fifth offer an explanation of how we arrive at this result, but I know it as a Tritone interval, am I wrong?

skinny peacock
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    Written as A > D#, it's an aug.4th. Written as A > Eb, it's a dim5th. Either way, it's going to be a tritone, but quoted here, it's more specifically the former. – Tim Mar 01 '18 at 14:55
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    Also note that an augmented 4th and a diminished 5th are the same pitch in equal temperament, but not necessarily in other tunings, so _tritone_ is less precise. –  Mar 01 '18 at 15:05
  • Although it's often convenient short-hard in casual settings when everyone knows more or less the context, technically speaking _tritone_ is not an interval name - just a measure of the distance between two notes. An _interval_ contains **musical information**. Augmented: a major or perfect interval increased by one additional half-step; 4th: The 4th note in the scale starting from the first note. – Stinkfoot Mar 02 '18 at 20:48
  • @Stinkfoot- I'll keep that in mind next time, it's good to know. – skinny peacock Mar 04 '18 at 05:29