good ˈevening formal
used to say hello when you are greeting someone in the evening, especially someone you do not know ⇒ good afternoon, good morning, good night

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In informal everyday English, people often just say Evening. If you don't want to sound formal or informal, you can just say Hello:
Evening, Tom.

[TahlilGaran] Dictionary of Contemporary English ▲

good evening
interj. Hello or goodbye.
■ Used as a formal greeting or salute when you meet or leave someone in the evening.
When the TV program began, an announcer appeared and said, "Good evening, everyone."
Finally Aunt May stood up and said, "I will not sell the house. Good evening, Mr. Flynn. "

[TahlilGaran] English Idioms Dictionary ▲