Market definition is the first step in nearly every antitrust merger analysis: identifying the relevant market in which the merging parties compete. Before regulators can measure whether a deal harms competition, they must define the arena — because market shares, concentration and competitive effects are all measured within a defined market. Market definition is frequently the most outcome-determinative issue in a merger case.
Two dimensions
A relevant market has two components:
- Product market — the set of products or services that customers view as reasonable substitutes for one another. (Are premium and economy brands in the same market? Are cable and streaming?)
- Geographic market — the geographic area over which competition occurs: local, regional, national or global, depending on where customers can practically turn for supply.
Together these answer: what does the merged firm really compete against?
The SSNIP / hypothetical-monopolist test
The standard analytical tool is the hypothetical monopolist test, operationalized as the SSNIP test ("Small but Significant Non-transitory Increase in Price," typically 5%). The question: if a single hypothetical monopolist controlled a candidate set of products, could it profitably impose a ~5% price increase? If customers would switch to other products in such numbers that the increase would be unprofitable, those other products belong in the market, and the candidate market is widened to include them. The market is the smallest set of products over which the hypothetical monopolist could profitably raise price.
Why it is so contested
Market definition is pivotal because it drives the concentration measure, and the two sides have opposite incentives:
- Regulators challenging a deal favor a narrow market — fewer competitors, higher combined shares, a higher HHI, and thus a stronger case for harm.
- Merging parties favor a broad market — more competitors and substitutes, lower shares, and an easier path to clearance.
A merger that looks anticompetitive in a narrow market may look benign in a broad one, so enormous analytical (and litigation) effort goes into where the line is drawn.
Its role and limits
Once the market is defined, regulators measure concentration and assess competitive effects under the Merger Guidelines. Notably, the 2023 U.S. Merger Guidelines and modern economic practice place somewhat less exclusive weight on formal market definition, allowing direct evidence of competitive effects (e.g., evidence the merging firms are each other's closest competitors) to carry the analysis even where market boundaries are fuzzy. Still, defining the relevant market remains the conventional starting point of merger review in both the U.S. and the EU.
See also
- Herfindahl-Hirschman Index — A measure of market concentration calculated as the sum of squared market shares. Used by U.S. and EU antitrust authorities as the primary screening metric in merger reviews.
- Antitrust and merger control — Government review of mergers to prevent harm to competition.
- FTC merger review — Competition review of a transaction by the U.S. Federal Trade Commission, sharing jurisdiction with the DOJ Antitrust Division for HSR-reportable deals.
- DOJ Antitrust Division review — Competition review by the U.S. Department of Justice Antitrust Division. Allocation between DOJ and FTC depends on the industries involved.
- EU Merger Regulation — Council Regulation (EC) No 139/2004, which gives the European Commission jurisdiction over mergers with an EU dimension. Deals above turnover thresholds are reviewed at EU level rather than by member states.