An Etymological Dictionary of Astronomy and Astrophysics

1. A piece of equipment consisting of a series of bars or steps between two upright lengths of wood, metal, or rope, used for climbing up or down something.
   

2. A series of ascending stages by which someone or something may progress (OxfordDictionaries.com).

Etymology (EN): M.E. laddre, O.E. hlæder “ladder, steps” (cognates: M.Du. ledere, O.H.G. leitara, Ger. Leiter), from PIE root *klei- “to lean,” → incline.

Etymology (PE): Nardebân “ladder.”