|The Simple English Wiktionary has a definition for: phenomenon.|
It comes from the noun φαινόμενον (phainomenon, df. appearance) and is also related to the verb φαινειν (phainein, df. to show). Its plural is phenomena. Its antonym is lathomenon, meaning "something lying beneath the surface".
General sense and use [change]
In general, apart from its original use as a term in philosophy, phenomenon stands for any observable event. Some events are easy to observe, others need scientific experiments that make observation possible.