We are writing a mathematical article. We are describing objects (unit squares) rotated only 0° and 45°.
For easier use we call 0° rotated "axial".
What should we call 45° rotated? Using just "rotated" can be misleading, as it could mean arbitrarily rotated.
"Oblique" seems best to me. "Diagonal" is something else.
Edit: Sure, we are going to define it. But imagine a casual reader who looks in the middle of the article and see "rotated square". It is neither obvious that this square is 45° rotated nor that there is some definition. "mitred square" does not sounds familiar and reader will see the definition.
Best Answer
Also consider calling the rotated square a biased square or a square on the bias, where bias imparts the sense of diagonal, oblique, slanting.
For example, on the bias ordinarily refers to a 45° slope (or more precisely, the slope of a series of intersections of warp and weft) in sewing. From Merriam Webster:
From OED1, sense A.1. of bias:
You might also refer to an indexed square. Like rotated square, the phrase indexed square does not imply a certain angle, such as 45°; but on the other hand, indexing is not free rotation. It frequently is to cardinal angles or to multiples of 15, 30, or 45°. In short, indexing an item means rotating or moving it to a preset location: