To speak with another is a dialogue, though in psychotherapy the dialogue assumes a particular flavour and intensity.
The subjective co-creation of a therapy makes every therapeutic session unique.
The therapist and the client create a mutually shared set of meanings based on the issues presented and explored by the client.
Another way of saying this is that they relate in a world of language, though there is also unspoken language to consider. Words take on meanings that best express the specific reality of the client.