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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".
Use in science & technology
In science, phenomenon stands for any event which we can sense or detect or record. In science, the word "observe" means more than just "visible to the naked eye". It means "sensible", available to our senses. The senses we are born with are extended by a whole range of instruments which record things we cannot see, and make them visible to us.
So, while some events are easy to observe, others need technology to make observation possible.