iudex

Iudex, iudicis is third declension masculine, and it is a compound you can take apart: ius, "law", plus the root of dico, "I speak". A iudex is literally a law-sayer. The same root gives iudicium, "a judgement or trial", and iudico, "I judge".

The trap is that a Roman iudex was usually not a professional magistrate but a private citizen appointed to hear a case, so in Republican contexts "juror" is often the better English. In the ecclesiastical and medieval corpus the word has settled into the modern sense, and God as iudex omnium is one of its commonest jobs. Note the spelling: the classical text has i where later manuscripts and modern editions often print j, so judex and iudex are the same word.