*au- Look up *au- at Dictionary.com
Proto-Indo-European root meaning "to perceive."

It forms all or part of: aesthete; aesthetic; anesthesia; audible; audience; audio; audio-; audit; audition; auditor; auditorium; auditory; hyperaesthesia; kinesthetic; oyer; oyez; obedient; obey; paraesthesia; synaesthesia.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit avih, Avestan avish "openly, evidently;" Greek aisthanesthai "to feel;" Latin audire "to hear;" Old Church Slavonic javiti "to reveal."