izvanredan

1. outstanding; excellent; remarkable
   Ona je izvanredna pevačica. — She is an outstanding singer.
   Večera je bila izvanredna, hvala vam. — The dinner was excellent, thank you.
2. extraordinary; exceptional
   To je bila izvanredna prilika za nas. — That was an exceptional opportunity for us.

For 'special, non-regular' (session, state of emergency) Serbian usually uses "vanredan"; vanredni profesor is an academic rank, roughly 'associate professor'.