What is the X in X-mas?
1570s, from French orthogonal, from orthogone, from Late Latin orthogonius, from Greek orthogonios "right-angled," from ortho- "straight" (see ortho-) + gonia "angle," related to gony "knee" (see knee (n.)). Related: Orthogonally.
Relating to or composed of right angles.
Relating to a matrix whose transpose equals its inverse.
Relating to a linear transformation that preserves the length of vectors.