Final answer:
To achieve economic efficiency, we strive for productive efficiency and allocative efficiency in any economic system. Perfectly competitive markets are unique in that they achieve both in the long run, whereas other market structures are considered imperfect if they lack one or both efficiencies.
Step-by-step explanation:
The two efficiencies that we try to achieve in any economic system are productive efficiency and allocative efficiency. Productive efficiency occurs when it is impossible to produce more of one good without decreasing the quantity that is produced of another good, indicating that all resources are being used to their fullest capacity. Allocative efficiency, on the other hand, occurs when the mix of goods being produced represents the allocation that society most desires, meaning that the optimal amount of each good and service is being produced and consumed.
In a perfectly competitive market, both productive and allocative efficiency are achieved in the long run, which is why it is referred to as 'perfect.' However, when analyzing other market structures, if they lack either of these efficiencies, we label them as 'imperfect.' For example, a monopoly may not produce at the lowest possible cost per unit (lacking productive efficiency) and often does not produce the optimal mix of goods and services that society desires (lacking allocative efficiency).