Which transmission between a "sending" device and a "receiving" device only allows a signal to move one way--ever?

Simplex is the proper answer to this question. By definition, simplex refers to a type of communication channel that only allows the transfer of information through one direction. This may be referred to as Duplex.

If in case you are not familiar with a duplex communication, this is a sort of point-to-point system wherein there are two or more connected parties that are available. In the case of using simplex, a half-duplex may be more appropriate.

The communication will signal will only flow towards one direction at a time. The difference of this with the simplex is that the direction can also go towards the other direction but not at the same time.