Efficient scale
The quantity at which ATC is minimum.
Excess capacity
Occurs when a firm produces below its efficient scale.
Markup
The amount by which the price exceeds marginal cost.
Monopolistic competition
A market structure in which: a large number of firms compete; each firm produces a differentiated product; firms compete on product price, quality, and marketing; firms are free to enter and exit.
Product differentiation
Making a product that is slightly different to those of competing firms.
Signal
An action taken by an informed person/firm to send a message to uninformed people.