arbitrium

Etymology
From.

Noun

 * 1) the decision of an arbiter, arbitration
 * 2) judgement, decision, opinion
 * 3) discretion, liberal decision; arbitrary decision, whim
 * 4) mastery, dominion, authority

Descendants
The forms that reflect /ī/ as opposed to the expected /i/ may be due to yod metathesis (/idrju > ijdru/), or indicate borrowings.