Old fashioned? No, I think you'll find that the latest books on the subject will say the same thing I said above. It beats the hell out of me why such a simple, straight-forward definition is so hard for people to accept. What's to be gained by saying something is not what it is? It just confuses things. I stick by what I said.

Two notes played together is called (are called?) an interval. The same term applies to two notes played consecutively. There are major and minor 2nds (C and D, C and Db), major and minor thirds (C and E, C and Eb), perfect 4ths (C and F), perfect 5ths (C and G), major and minor 6ths (C and A, C and Ab), major and minor 7ths (C and B, C and Bb), and octaves (C and C).

Perfect intervals can also be augmented and diminished, i.e., augmented and diminished 4th (C and Fb [in even temperment, same sound as C and E, a major 3rd], C and F#). Augmented and diminished 5th (C and G#, C and Gb [this is sometimes called a "tritone"--this interval is an essential ingredient in a dominant 7th chord, and it's also the famous "flatted 5th" in jazz. It has a very interesting history]).

Try playing these intervals, together and consecutively. It's a revealing experience to hear the sound and be able to put a name to it.

Don Firth