Economies of scale are achieved when increasing the scale of production decreases long-term average costs. In other words, the cost of production per unit decreases as a company produces more units.
Diseconomies of scale are the product of decreasing returns to scale. In other words, they happen when a business grows to the point that its per-unit costs begin to rise, rather than continuing to decrease as with economies of scale.
Perfect competition or pure competition is a type of market structure. It is important to note that this form of market structure does not actually exist in the real world and is thus considered to be theoretical.
In an oligopoly market structure, there are just a few interdependent firms that collectively dominate the market. While individually powerful, each of these firms also cannot prevent other competing firms from holding sway over the market.
In monopolistic competition, there are many small firms who all have minimal shares of the market. Firms have many competitors, but each one sells a slightly different product.
The marginal cost of production is an economic concept that describes the increase in total production cost when producing one more unit of a good. It is highly useful to decision-making in that it allows firms to understand what level of production will allow them to have economies of scale.