A witness is defined as anyone who can present evidence in a case ("Witness," West's Encyclopedia of American Law, 2nd. ed.). In Harper Lee's To Kill a Mockingbird, Tom Robinson's trial was extremely unusual due to lack of evidence and the fact that only two out of four people who testified at the trial can genuinely be considered witnesses, though the plaintiff and the defendant of a case certainly can testify as witnesses.
In the case, Mayella Ewell is considered the plaintiff, the person who is accusing another of wrongdoing. Tom Robinson is considered the defendant, the person who is being accused of a crime or offense and is denying involvement (West's Encyclopedia of American Law, 2nd. ed.). Both the plaintiff and the defendant certainly can serve as witnesses by taking the witness stand to state their own testimonies of what occurred; however, their testimonies are given less weight than the testimonies of actual witnesses, and the burden of proof always rests on the plaintiff. In other words, it would have been Mayella's responsibility to prove the crime actually took place, and her own testimony alone would not serve as proof. In the case, both Mayella and Robinson served as witnesses by taking the stand. While it is more common for plaintiffs to take the witness stand, it is less common for defendants to take the witness stand, and defendants are Constitutionally protected from having to do so. Often, a defense lawyer will not call a defendant to the witness stand should he/she say something that damages the case under cross-examination by the prosecuting attorney. However, in this case, Atticus felt he had no witness to call other than Robinson himself.
One key witness in the trial is Sheriff Heck Tate, who testifies having seen Mayella looking very beaten up. Sheriff Tate gives two crucial statements that give Atticus grounds to motion to dismiss the case, a motion he sadly does not make. Sheriff's first crucial statement is that a doctor was not summoned to examine Mayella on the evening in question. Sheriff Tate states the following to Atticus in defense of having not called a doctor:
It wasn't necessary, Mr. Finch. She was mighty banged up. Something sho' happened, it was obvious. (Ch. 16)
However, Mayella isn't just accusing Robinson of having abused her; she is accusing him of having raped her, and based on the principle corpus delicti, meaning body of the crime, it is illegal to try a defendant without concrete evidence that the crime actually took place. A doctor's testimony would be essential for Mayella's proof, and without such proof, the case could have legally been dismissed. The second crucial statement Sheriff Tate makes is that Mayella was bruised in her right eye, which could have only been accomplished by a left-handed person facing her. Since Robinson is crippled in his left arm and hand, this second statement of Sherrif Tate's also gives Atticus grounds to motion to dismiss the case, which he sadly does not do.
Last, Bob Ewell, Mayella's father, serves as the only eyewitness in the trial. He testifies hearing Mayella scream inside the house and running up to the window to see "that black nigger yonder ruttin' on my Mayella" (Ch. 16). However, Ewell's testimony is later contradicted by Mayella's own testimony, in which says she saw her father "standing over [her] hollerin' who done it, who done it?" (Ch. 18). If Ewell had truly witnessed the event, he would have had no reason to ask Mayella who the culprit was, which helps to expose the Ewells' lies.