The US Senate's impeachment trial of President Donald Trump begins today, and there's one term that's already played an important part in the process: hearsay. WGBH News' Morning Edition host Joe Mathieu spoke with Northeastern University law professor and WGBH News legal analyst Daniel Medwed about what the term legally means. The transcript has been edited for clarity.

Joe Mathieu: This is like the day of days for people in the legal world. My gosh, we have a Senate impeachment trial. And it's unclear if additional evidence is going to be allowed in the Senate. But let's start with the basics, Daniel. What exactly is hearsay evidence?

Daniel Medwed: The issue of hearsay arises when one side wants to introduce evidence about what a person heard someone else say to prove a point. I think the best way to illustrate this concept is through an example. Let's say I'm on trial for being a bad teacher. Hard to believe, I imagine! The opponent tries to introduce evidence that one of my former students last week told a friend that Medwed is incompetent in the classroom. That's classic hearsay: it's an out of court statement being offered to prove the truth of the matter asserted in the statement — that I'm incompetent in the classroom. And under the rules of evidence, that hearsay is kept out.

Mathieu: So what's the rationale for keeping it out? After all, you would think what a former student said about the teacher would be quite relevant in a case against you for being a poor teacher.

Medwed: Oh, that's right. Much of hearsay is relevant. The question is, is it reliable? It's coming from out of court. It's proving an important point. We need to have mechanisms to ensure veracity. Let's juxtapose that with in-court live witness testimony where there are lots of protections. First, there's the oath. A witness has to swear to testify to the truth under penalties of perjury. Second, there's cross-examination, which people have called the greatest legal engine ever devised for the discovery of the truth. And finally, jurors can evaluate demeanor. They can look at the witness, see whether she's fidgeting, whether she's averting her eyes, whether she's using evasive language. There are all those mechanisms to gauge credibility. They don't necessarily exist in the hearsay context, which means that even relevant evidence might be excluded as unreliable.

Mathieu: Hence, Democrats interest in adding witnesses at this stage of the process.

Medwed: Absolutely.

Mathieu: Is all hearsay evidence inadmissible in court? Are there exceptions, Daniel?

Medwed: Well, there are lots of exceptions. In fact, there are dozens and dozens of exceptions where a particular piece of hearsay is deemed especially reliable and or necessary. Here are a couple examples. Let's take former testimony. A person testifies at a hearing or a past trial, then disappears before the current proceeding. Their previous statement can come in as a hearsay exception, as former testimony in the current proceeding because at the time that original statement was taken, it had the trappings of reliability, it was given under oath and so on. Other exceptions are more controversial. There's something called the excited utterance exception. You observe an exciting event — "Look, Joe! There's a UFO outside the window!" And the idea is that exciting event has somehow stilled or stopped your capacity to fabricate; you're reacting so quickly that you can't conjure up a lie. And the rule writers would say that hearsay can come into court later.

Mathieu: I'm guessing there are even more interesting exceptions here when it comes to the legal world.

Medwed: Oh, my favorite is called "the dying declaration." People probably seen this in old movies. Somebody believes their death is imminent. They make a statement about the cause or circumstances of their looming death. That statement can come in in a future homicide trial, even if they don't appear at all, even if they die. Now, on the one hand, the rationale really is a religious one -- this idea that you don't want to meet your maker with a lie on your lips, or as I think Shakespeare said, "[where] words are scarce, they are seldom spent in vain." But on the other hand, a lot of research suggests that when you're on the cusp of death, when you're on death's door, you're suffering a lot of physical and psychological trauma. You might say you might be delirious. You might say anything.