In marketing, a Conquest Sale is a term describing a sale to a particularly hotly contested customer, who does not have a specific reason for shopping at one store over another, or for purchasing a particular item or service over another.
Imagine, for a second, that Coca-Cola and Pepsi Cola are identical. They generally cost the same, they taste the same, and they are sold in the same stores. The sale of a hundred bottles of coke means that the Coca-Cola company earns X dollars - nothing special about that. However, this is a Conquest Sale: A thirsty customer who wants to quench their thirst, will buy a single cola-type beverage: They will buy either a bottle of Pepsi or a bottle of Coca-Cola. Because they only need a single bottle to become un-thirsty, the customer is highly unlikely to buy both.
This means that if Coke manages to sell 100 bottles of coke, not only has it earned X dollars, it has also deprived Pepsi of the same amount of dollars. On the sales budget, therefore, the sale of 100 bottles of coke is a simultaneous loss to the competitor: In addition to being 100 dollars up on their own budgets, the Coca-Cola company would have contributed to 100 dollars of loss to the Pepsi company. In the grand scheme of things, then, this puts Coca-Cola 200 dollars ahead of Pepsi. In a competitive market place like carbonated beverages, this is an important achievement.
This is true in all commodity markets, but is more markedly noted in markets where there are two or three large competitors. Ford and Chevrolet trucks, for example, can be argued to be more or less identical, at more or less the same price. Getting a customer to buy a Ford truck as opposed to any other car is a good sale. Getting a customer to buy a Ford truck instead of a Chevrolet truck is a conquest sale.
Another good example is Intel and AMD: you can only have 1 type of CPU in a single PC. According to a source who works in computer processor chips sales, it all heated up in the last few years and this year's marketing theatre is already looking to be very exciting.
It is all about being a thorn in the eye of the competition.