A state effectively dominated by two or more major powers, and open to military and political annexation by any of them at will, but left intentionally independent, in the interest of providing a neutral "cushion" between the great powers.
Buffer states may be old but weak states, ripe for conquest, but left nominally independent - e.g. Denmark in 1864. They may also be young or even newly-created states, formed to give the appearance of a less expansionist policy on behalf of the dominant nation(s) - e.g. the Duchy of Warsaw, formed after the Treaty of Tilsit, 1807.
Such a construction is often the mark of an uneasy peace - and of recognition that a shared border is an invitation to border incidents which might conceivably escalate into outright war. Under the circumstances, the major powers choose to forego formally annexing the state, in order to secure stability. Political propaganda issues may also play a part - there may be more to be gained by letting a weak state persist than by annexing it, in terms of propaganda.
Related concepts: no man's land, buffer zone.
Other example(s): Finland during the Cold War, Westphalia and the Septinsular Republic in the Napoleonic Wars period.