In the ongoing antitrust trial between Google and the US Justice Department, the tech giant's defense strategy hinges on defining the market for online advertising technology. The trial commenced on Monday in an Alexandria, Va., courtroom and holds significant implications for the future of the multibillion-dollar industry.

Former DOJ antitrust attorney, Dan McCuaig, emphasized the importance of market definition in antitrust cases, stating that it has the potential to be "outcome determinative." Google's ability to prove that the technologies behind online advertising constitute a single, broader market could be vital in countering allegations of illegal monopoly.

This trial follows Google's recent defeat in an antitrust case regarding its monopolization of the online search engine market. The company now faces claims from federal prosecutors who argue that it illegally monopolized three distinct markets related to open web display ads: the publishing ad server market, the ad exchange market, and the advertiser ad network market. These technologies account for over $24 billion in annual online ad sales.

During the trial's opening statements, Google's lawyer, Karen Dunn, accused the DOJ of attempting to divide the market by eliminating substitute online advertising services. She argued that market definition should encompass not only algorithm-driven auctions but also human-to-human negotiations, making ad tech tools part of a larger online advertising market.

While Google asserts that it competes with various tech giants like Microsoft, Facebook, Amazon, TikTok, Roku, Disney, and Yahoo Finance's parent company, Yahoo, the government's first witnesses claimed the existence of separate ad tech markets. James Avery, founder and CEO of ad tech company Kevel, testified that his company solely serves the publisher side of the industry and faced obstacles when competing with Google's proprietary technology.

Additionally, witness Joshua Lowcock, president of advertising data firm Quad, highlighted the uniqueness of open web display ads that can be placed anywhere on the internet, distinguishing them from platform-specific ad auctions. He also acknowledged the presence of "direct" transactions negotiated between people as a separate market.

Contradicting the government's stance, Google's online advertising competitors, including Microsoft, TikTok, and Yahoo, were seen by antitrust expert Alden Abbott as barriers to proving antitrust harm. Abbott argued that the absence of consumer harm and independent consumer demand for publisher- and advertiser-side technologies weaken the DOJ's case.

The trial's outcome will determine the extent of Google's influence in the online advertising technology market and will have far-reaching implications for antitrust regulations. As the trial continues, Google will have the opportunity to present its own witnesses once the government rests its case. The ultimate judgment will also depend on the welfare of consumers and whether the DOJ can demonstrate harm caused by Google's alleged monopolistic practices.

Overall, the trial raises fundamental questions about market definition and the enforcement of antitrust laws in the rapidly evolving field of online advertising technology.