Why is it called half-duplex?

Answer 1

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As far as I can tell, the terms 'simplex', 'half-duplex', and 'full duplex' originated with the Western Union Telegram company.

I'm not sure how they came up with the terms, but 'simplex' refers to a system that can send data, but not receive (like a radio station), 'half-duplex' is a system that can send and receive - but only one at a time (like a two-way radio), and 'full-duplex' allows simultaneously two-way communication (telephone).

Answer 2

because it does half of what full duplex does:

https://en.m.wikipedia.org/wiki/Duplex_(telecommun...