Is TCP bidirectional or full-duplex?

Question

Bidirectional and full-duplex are different concepts. For example the Ethernet is only half-duplex because at a specific time, only on

Answer 1

It is both. It is bidirectional because it can send data in both directions, and it is full-duplex because it can do that simultaneously, without requiring line turnarounds, at the API level.

Of course at a lower level it may be restricted by the available physical layer.