Two-way simultaneous communications is a term sometimes used to disambiguate the term duplex (telecommunications). In a two-way simultaneous communications channel, each endpoint can both send and receive at the same time. Two-way alternating communications allows an endpoint either to talk or listen, but not both, at any given time.
Two-way simultaneous communications are sometimes called full-duplex, but there may be a fine distinction that two-way alternating communications is a property of the communication protocol used between the endpoints, while the underlying communications medium may support either two-way alternating or two-way simultaneous communications. When the underlying communications medium provides two physical paths, one for each direction of communications, or has some mechanism for providing two logical paths, such as multiplexing or packet-switching, the medium is full-duplex.