Dialectic is an open-ended process of critical thinking by which opposing voices or positions in a dialogue or debate may include differing points of view yet move forward with a sense of progress.
Oxford English Dictionary 2a: In modern Philosophy . . . by Hegel . . . the term is applied (a) to the process of thought by which such contradictions are seen to merge themselves in a higher truth that comprehends them . . .
b. In more general use, the existence or working of opposing forces, tendencies, etc. Also in pl. form [dialectics]
Most familiar form: "the Hegelian Dialectic":
thesis + antithesis => synthesis
The thesis (standpoint, opinion, idea) meets its anti-thesis (opposiition, counter-opinion, counter-point). As the two interact, they find common ground in a synthesis, fusion, or compromise, which then becomes its own thesis or idea, and the process continues.
In common practice and usage, "the dialectic" describes civil or intellectual dialogue of opposing ideas, opinions, or positions as they shape modern thought and politics in a free or open state, in faith that people who talk with and listen to each other can meet to solve pressing problems or at least not fight.
Person or Party A proposes an idea or policy, leading Party B—"the loyal opposition"—to respond by pointing out problems with the idea (or thesis) advanced.
Party A then responds to Party B by conceding whatever validity there may be to Party B's objections, and changing Proposal A accordingly.
Party B may then accept the compromise, or the process may be repeated until completion or until some limit.
In brief, the essential speech-form of dialectic is "yes, but . . . ."
The dialectic is originally modeled by the Socratic Dialogue or Socratic Method, in which individuals with opposing ideas ask and answer questions of each other as a means of critical thinking and enlightened development of ideas.