Defining competition in a U.S. Courtroom involves the analytical and intellectual collision of the laws pragmatic aspects with the academic realities of economics. Both disciplines depend heavily upon competition, and employ a rich dosage of competition language. However, competition in law and competition in economics are dramatically different.Economists often study market efficiencies. In an academic setting, economics and econometrics evaluate efficiency, and assess its achievement or failure. As a social science, the study of markets by economists often involves the specific assessment of market efficiencies. Here, too, the laws social disciplines differ greatly from those of academic economics. Except for a few aberrant moments of brief duration, the process of making, enforcing, and litigating over legal principles in historys democracies has never involved pursuit of an efficient economy, or even an efficient legal system. To the contrary, the laws goal is to govern behavior to ensure fairness, justice, legal compliance, and not efficiency.Through analysis of a history-making U.S. cattle market trial, this paper considers legal proof and illustrates application of the rules of evidence and courtroom-level definitions of proof and evidence. Routinely, juries are instructed on what constitutes proof, and what does not. In the legal case that provides this papers illustrative focus, the United States District Courts definition of evidence for the jurors, the courts rulings on evidence issues, and the lawyers arguments of the evidence to the jury impacted an entire industry. The case provides a useful tool for studying and defining competition in a U.S. courtroom.