What does the ~ (tilde) mean for pins like "~RESET"?

The ~ in pins names like ~RST means not.

A pin named without ~, for example RST or RESET, will perform its function (e.g. reset the device) when it is connected to VCC
A pin named with ~, for example ~RST or ~RESET, will perform its function (e.g. reset the device) when it is connected to GND

$$\overline{\text{RESET}}$$$$\overline{\text{RESET}}$$

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