What is the definition of assembly line justice?
Assembly line justice is not really a technical term. Instead, it is a pejorative term that people use when they think that the courts are not really giving defendants a fair hearing. To relate it to previous questions of yours, it is what an advocate of the due process model would say to criticize the court system.
Assembly line justice is a system in which the defendant's rights are not rigorously protected. It is a system where defendants get rushed through the system as quickly as they can be. They do not get to really consult with attorneys, build a defense, etc. They might meet their attorney for the first time when they get to court for their trial.
Overall, then, assembly line justice is a term that describes a system that does not (in some people's minds) pay enough attention to the rights of the accused and instead tries to process them through their trials and find them guilty as quickly as possible.
It probably depends who you ask, but I would define it as a court system that overwhelmingly uses plea bargains as a means of settling cases, so that the time and expense of a trial is avoided, some measure of punitive sentencing can be guaranteed, and so a system can process many, many more cases than it would ordinarily be able to. Cases, in other words, receive only superficial attention.
Owing largely to the huge number of drug cases now before US courts, something we have only witnessed during the Prohibition Era and the last forty years of the Drug War, many people would argue the US justice system has devolved into assembly line justice, in the plea bargain fashion, and is unrecognizable in the modern day.
Besides being faster and cheaper, it also saves the witnesses from having to testify, which is often for the best.