Lewis Short
observÄtor (noun M) : id.
* A watcher, observer (perh. not ante-Aug.): nemo observator, nemo castigator assistet,Plin. Pan. 40: observator et custos bonorum,Sen. Ep. 41, 2: Catholicae legis, i. e. one who obeys it, Cod. Th. 16, 5, 1.
Charlton T. Lewis, Charles Short, A Latin Dictionary