The End Of Text character (ETX) is an ASCII control character used to inform the receiving computer that the end of the data stream has been reached. This may or may not be an indication that all of the data has been received.
Often used as a "break" character (Control-C) to interrupt a program or process. In TOPS-20, it was used to gain the system's attention before logging in.
It is often used in conjunction with Start of text (STX) and Data Link Escape (DLE) e.g. to distinguish frames in the Data link layer.