
American Analytic philosopher David K. Lewis wrote the book on convention. By considering the meaning of convention, Dr. Lewis brings insight into several important areas with particular interest paid to language. The book was inspired by a spirited attack on the idea that language is ruled by convention by Lewis's teacher WVO Quine. Quine's questions can be paraphrased: "If language is ruled by convention, what language was the convention in? If convention is not literal, what is meant by it? If this is a vauge metaphor, what makes us think it's the right one?". Lewis was also inspired by economist Thomas Schelling's analysis of Coordination Problems.

I do not envy the person who had to come up with a cover to a book titled Convention, but I cannot praise their total lack of effort.
The model that the remaining definitions is - unless stated otherwise - the
Normal-Form Game of Von Neumann and Morgenstern. A normal form game can be characterized by it's demonstration of payoffs for different options. It represents each player as a dimension and each option as a row, column or higher dimensional axis. One of the classics is it's representation of the Prisoner's Dilemma:

An alternate way of characterizing Normal-Form games is as functions. The set (lets say of size n) of players puts their choice into a
n-tuple, which is then fed into the function. The function maps the decisions (which are integers) onto payouts (which can be real). In the case of the above game P(<"Player One","Player Two">) = <"bottom number","top number">. So to completely characterize this function P: P(<1,1>) = <3,3>; P(<1,2>) = <5,0>; P(<2,1>) = <0,5>; and P(<1,1>) = <2,2>.
Most of the mathematical details are inessential to the analysis - Lewis goes as far as to say that he could remove it entirely without substantially weakening his position. Lewis mainly uses
Equilibria as his tool for insight, allowing for strong conclusions and good generality. Following Schelling, Lewis distinguishes a continuum between games of pure coordination - games in which the player's payoffs are equal in every square - and games of pure competition - games in which the player's payoffs are opposites in every square. Again, inspired by the work of Schelling Lewis restricts himself to games closer to cooperation.
An equilibrium is when each player has made the best choice given everyone else's choice. In other words, they could not have improved things by altering their and only their choice. In the Prisoner's Dilemma above, for example, Player One's payouts are either 3 or 0 if he chooses column one, and either 5 or 2 if he chooses column two. Thus column two is the better option no matter what Player Two does. Similarly, row two is better for Player Two no matter what Player One does. Thus <2,2> is the equilibrium.
Lewis is particularly interested in a subset of equilibria: coordination equilibria. A coordinition equilibrium is when each player would not have been better off if any player acted differently. That equilibria and coordinition equilibria are different may not be immeadiately apparent. In fact, in games of pure coordination they are identical (this is shown easily). However, they can be distinguished in games like the following example:

In this game <1,1>, <1,2>, <2,1>, and <2,2> are all the possible moves. <1,1> is a equilibrium - Player One prefers to could not improve by altering their choice, neither could Player Two improve by altering their choice - and a coordination equilibrium - Neither Player would be better off if any player changed. <1,2> and <2,1> are not equilibria - in both cases, one of the players (Player One in the case of <1,2> and Player Two in the case of <2,1>) would have been better off had they chosen differently. Thus neither are they coordination equilibrium. <2,2> is different. <2,2> is an equilibrium: Player One could not have made their lot better by changing to <1,2> and Player Two could not have made their lot better by changing to <2,1>. However, it is not a coordination equilibrium: Player One would be better off if Player 2 changed their decision (i.e. at <2,1>), and Player Two would be better off if Player One changed their decision (i.e. at <1,2>).
After some interesting analysis, Lewis proposes that "We may achieve coordination [equilibrium] by acting on concordant expectations about each other's action." (p. 27).
The next concept is that of a Regularity. A regularity is a pattern of behavior, a repetition of a decision. For instance, Player One (a population of size 1) might have an arbitrary preference in repeated Normal Form Games to choose option 1. A regularity can be in an individual or in a population. Regularities are relative to Situations. A Situation is anytime a choice must be made - we are describing these in terms of Normal Form Games.
A regularity in a population is a Convention if and only if 1) everyone in a population exhibits the regularity, 2) everyone expects everyone else in a population to exhibit the regularity, 3) everyone has approximately the same preferences in options (in other words, the situation is more cooperative than competitive), 4) everyone in the population prefers to exhibit the regularity on the condition that at least all but one person in the population also exhibits the regularity, and 5) there exists another possible regularity that matches rule 4 and it is impossible for one to follow both regularities at the same time.
To return to our second example, "Choose row/column one" might be a convention. Player One will choose column one. Player One thinks that Player Two will choose the row with the highest number for their payout, and thus have reason to expect Player Two will choose row one. Both want the highest payout. Both would prefer to choose row/column two if the other chose column/row two. Player One prefers to choose row one in any situation (payoffs are 1 or 0, which is better than .5 or .2) and the same with Player Two - thus they prefer to choose row/column one if at least all but one person in the population also chooses row/column one. Thus if Player Two chooses column one, the players have formed a convention - and without communication!
This is Lewis's definition of convention. I'm working on a defence/critique of it, but it's more involved than my usual posts, so I'm going to separate it into a new post. Stand by, it might be done before the day is out!