D defs.my
Entry 1 sense Webster, 1913

Rationale

/(răsh`ŭ*năl" [or] răsh`ŭn*ā"lē̇)/ · Ra·tion·a·le · IPA /ˌɹæʃ.əˈnæl/
01 n. An explanation or exposition of the principles of some opinion, action, hypothesis, phenomenon, or the like; also, the principles themselves.
  1. 1.
    An explanation or exposition of the principles of some opinion, action, hypothesis, phenomenon, or the like; also, the principles themselves.