Most steps could be said to be diagonal, on that basis. Several reputable published authors (David Turner, Christine Deniston & others) would say that backward walks are, effectively, taken on one track, not two, requiring every step to be diagonal to a small degree, but we would not generally think of them in that way, even if such a movement satisfied a 'dictionary definition' of the word. Perhaps you should address your own rhetorical quotation : what IS diagonal? I originally cited Ballroom technique as saying that it is as much to the side as forward or back (ie 45 degrees to the reference line, whatever it is). I accept that such a definition doesn't 'rule' here, but goodness, neither does the personal view of any individual poster who wishes to tell me off for my impertinence (and that is not a reference to you) ... The dictionary is clear. Diagonal is a straight line between the opposite corners of any rectangular shape, so the angle can be anything between a tiny fraction more than 0, or a tiny fraction less than 90. That isn't terribly helpful.