Shot reverse shot (or shot/countershot) is a film technique wherein one character is shown looking at another character (often off-screen), and then the other character is shown looking "back" at the first character. Since the characters are shown facing in opposite directions, the viewer assumes that they are looking at each other.
Shot reverse shot is a feature of the "classical" Hollywood style of continuity editing, which deemphasizes transitions between shots such that the audience perceives one continuous action that develops linearly, chronologically, and logically. It is in fact an example of an eyeline match.