An agreement between two or more actual or potential competitors operating at the same level of production or distribution in the market. For example, an agreement between two or more manufacturers or two or more retailers. Compare this to a vertical agreement. The distinction between the two is important under antitrust laws, because the nature of the relationship between the parties may determine how potential violations are evaluated.

For examples, see Practice Note, US Antitrust Laws: Overview: Horizontal or Vertical Restraints.