WordWeb Online

1. Not pertinent to the matter under consideration
   "an issue orthogonal to the debate";
   - extraneous, immaterial, impertinent
    
2. Statistically unrelated
    
3. Having a set of mutually perpendicular axes; meeting at right angles
   "wind and sea may displace the ship's centre of gravity along three orthogonal axes";
   - rectangular

See also: beside the point, irrelevant, perpendicular, unrelated

Encyclopedia: Orthogonal