A dialectician is a philosopher who views the world in terms of complementary opposites and the interactions thereof. In popular usage, the central feature of dialectic is the concept of "thesis, antithesis, synthesis" - when an idea or phenomenon (thesis) arises, it carries within itself the seed of its opposite (antithesis), and the interplay of these polarities leads to a synthesis which is somehow beyond the scope of either polarity alone. In turn, the synthesis is now itself a new thesis, and the entire process can begin again.
Dialecticians sometimes refer to this process as "the negation of the negation," meaning that as soon as the contradiction between thesis and antithesis is resolved by synthesis, the fact that a new thesis has emerged gives rise to a new antithesis and therefore another contradiction. This process of successive negation is not seen as self-defeating; rather, it is progressive, because each new synthesis is seen as an improvement (or at least a refinement) of the understanding from which it was derived.
Historically, dialecticians and dialectical thought have been primarily associated with Marxism, as the philosophical grounding of Marxism is based on a materialist interpretation of Hegelian dialectic. However, individuals widely recognized as dialecticians exist outside of Marxism. Indeed, dialectic is at least as old as Plato, who argues that it is the process by which one ascends the divided line (cf. "Republic" book VI) to reach the "unhypothetical first principle of everything." Plotinus, too, argued that dialectic was necessary in order to become Intellect, the second hypostasis, in the soul's search for the One.