Answer: Strictly speaking, it's an augmented fourth (such as C up to F#), and is called that because it's three whole scale tones in succession (for example, C-D, D-E, E-F#). But a diminished fifth (such as F# up to C) is just a tritone turned upside-down (inverted) and is treated in traditional music with the same care as the tritone. The tritone was in traditional counterpoint studies known as "the devil in music," and was avoided as a difficult-to-sing-in-tune awkward melodic interval. You may read stories at unreliable internet sources about this interval being "banned by the Catholic Church" but really it just presented a technical problem in composition and performance, and so the style "rules" studied by budding musicians advised that one should not use it. Nonetheless, it was used on occasion, and some theorists thought it useful when handled well.

Leonard Bernstein made a little joke out of this in West Side Story - in particular with his song "Maria," whose melody begins with a tritone, Eb-A, that is lovingly repeated.

If you're not sure how to identify an augmented fourth (or any other interval) remember that Practica Musica will identify any interval for you if you just play the notes on the screen piano when in Practice mode. Or enter the notes on the staff (same thing works for chords). If you need to choose specific flats or sharps you might find it easiest to use the "enharmonic keyboard" in which each key is divided up into sharps and flats.

More history, whether you want it or not: a line used to remember the tritone rule was "Mi contra Fa, Diabolus in Musica." That's the origin of the "devil in music" story; this has nothing to do with the church. The Mi and Fa don't refer to the third and fourth notes of a modern major scale (as in Do, Re, Mi, Fa), either: the reference goes back to medieval use of overlapping hexachords (six note scales) as an organizing principle for the gamut of tones that make up music. The Mi of any particular hexachord forms a tritone with the Fa of the next following hexachord. For example, the hexachord on G is followed by the hexachord on C, and the Mi of the G hexachord (B) forms a tritone with the Fa of the C hexachord (F). Similarly, the Mi of the C hexachord (E) forms a tritone with the Fa (Bb)of the next hexachord, which begins on F. There's an excellent article on the Tritone at Wikipedia that will provide as much detail as anyone could desire.