A more thorough explanation:

Primary-line injury is a term used in antitrust law to describe the practice of charging below-cost prices to eliminate competition in the market. This is a violation of the Robinson-Patman Act, which prohibits price discrimination.

There are two types of primary-line injury:

• Primary-line injury: This type of injury hinders or seeks to hinder competition among the seller's competitors. For example, if a company sells a product at a price lower than its cost to produce it, in an attempt to drive its competitors out of business, this is a primary-line injury.
• Secondary-line injury: This type of injury refers to discriminatory pricing that hinders or seeks to hinder competition among the seller's customers. For example, if a company offers discounts to certain customers, but not to others, in an attempt to favor one customer over another, this is a secondary-line injury.

Here's an example:

Liggett, a cigarette manufacturer, accused Brown & Williamson of offering discriminatory volume rebates to wholesalers in an attempt to eliminate competition from the economy segment of the cigarette market. This is an example of a primary-line injury because it harms direct competitors of the discriminating seller.