A priori is a Latin term meaning "derived from the former" or "from the cause to the effect." People better understand the term when they hear it is synonymous with deductive reasoning. It is the idea that certain things can be deduced and accepted without needing a significant amount of proof, from former events that have occurred. This applies to evidence presented in a court, or precepts a court holds about how to interpret law. In either case, anything deduced is regarded as not needing to be proven with additional experimentation or evidence; it logically proceeds from some former knowledge.
There are some kinds of evidence that can be admitted without needing to prove its validity. For example, one witness has perhaps happened upon a dead body. There are a priori assumptions that go immediately with this, that the person who was killed is dead. This may not require much additional proof.
When a prosecutor puts together all the evidence for a trial, there might be certain a priori evidence that comes along with this. Some facts are automatically admitted by deductive assumption and others need to be proven. Attorneys have to evaluate all evidence and find necessary proofs for evidence not based on deductive and easy to assume reasoning from the facts that precede it.
Not all evidence is a priori; some things are much more obvious. A witness to a murder is not presenting a priori evidence but is confirming details that a crime occurred. The witness doesn’t deduce the murder if he saw it first hand. He could conclude he is witnessing a murder as opposed to an accident, and many things about what the witness saw could make this a logical deduction.
Other forms of evidence are more difficult to deduce, and require lots multiple proofs before they can be admitted in court. This may mean having witnesses that can tell the same story about events, or using a variety of experts to support conclusions that are made about what happened during a crime. Sometimes it’s even necessary to provide evidence that witnesses are qualified to testify, instead of deducing this is the case.
Acceptance that forensic science works in a specific way, without experimenting to prove it, could be an example of a priori acceptance by jurors and sometimes the courts. Defendants may be free to challenge this and offer expert witnesses to discredit testimony that is assumed scientific and thus without error. They may provide their own experts to prove that “expert testimony” isn't proof of a deduced scenario or that assumption of science's perfect conclusions is incorrect.
The term a priori is not always used positively in law, and can be an accusation leveled at opposing attorneys. Instead of meaning logically deduced, it could be mean that it is unsupported by other evidence. If an attorney argues that someone else's evidence is deduced only, he may be stating that opponents haven't provided enough proof to support a deduction, and that such assumptions should be inadmissible.