acknowledgement,
acknowledgment a statement acknowledging something or someone; "she must have seen him but she gave no sign of acknowledgment"; "the preface contained an acknowledgment of those who had helped her"
Greeting is an act of communication in which human beings (as well as other members of the animal kingdom) intentionally make their presence known to each other, to show attention to, and to suggest a type of relationship or social status between individuals or groups of people coming in contact with each other. While greeting customs are highly culture- and situation-specific and may change within a culture depending on social status and relationship, they exist in all known human cultures. Greetings can be expressed both audibly and physically, and often involve a combination of the two.
OmegaWiki Dictionary
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greeting A conventional phrase used to start a conversation or otherwise to acknowledge a person's arrival or presence.
greeting A conventional phrase used to start a letter or other written communication.