Prejudice to the defendant refers to the harm or disadvantage that a defendant may suffer as a result of a delay in bringing a legal action, particularly in trademark cases. It highlights how such delays can affect a defendant's ability to mount an effective defense, often leading to an unfair situation where they cannot adequately respond to claims due to lost evidence, faded memories, or changing market conditions. This concept plays a critical role in legal defenses such as laches, estoppel, and acquiescence.