rationale

noun3

1. •
   An explanation of the basis or fundamental reasons for something.
2. •
   A justification or rationalization for something.
3. •
   (rare) A liturgical vestment worn by some Christian bishops of various denominations.

   • the local bishop, H.E. Gregor Maria Hanke, is shown wearing that rarest of liturgical garments, the rationale. — 2015