An Etymological Dictionary of Astronomy and Astrophysics

1. To make plain or clear; to render understandable or intelligible.
   

2. To make clear the cause or reason of; to account for.
   

3. To assign a meaning to; → interpret.

Etymology (EN): From L. explanare “to make level, smooth out; make clear,” from → ex- “out” + planus “flat; clear.”

Etymology (PE): Ostâhidan, literally “to unfold, spread out” (on the model of Fr. expliquer, Sp. explicar, from L. → ex- “out”

• plicare “to lay, fold, twist,” from PIE base *plek- “to plait, twist;” cf. Gk. plekein “to plait,” L. plectere “to plait, braid, intertwine,” O.C.S. plesti “to braid, plait, twist,” Goth. flahta “braid”), from os- “out,” → ex- + tâh “fold, plait, ply;”
  Mid.Pers. tâg “piece, part” + -idan infinitive suffix.