You know you are on HN when hello/hi is explained in terms of SYN/ACK and not the other way around... Jokes aside this is actually called a "phatic expression" in linguistics:
> In linguistics, a phatic expression is communication which serves to establish or maintain social relationships rather than to impart information. Phatic expressions are a socio-pragmatic function and are used in everyday conversational exchange typically expressed in situational instances that call for social cues [1]
> In linguistics, a phatic expression is communication which serves to establish or maintain social relationships rather than to impart information. Phatic expressions are a socio-pragmatic function and are used in everyday conversational exchange typically expressed in situational instances that call for social cues [1]
[1] https://en.wikipedia.org/wiki/Phatic_expression