Is there a formal definition of what is meant by “eyewitness” when the term is used in catholic apologetics?
I would define an “eyewitness” the same way you would in a courtroom generally-- someone who can testify about an event because they perceived it directly via one of the five senses. The Gospels are often described/defended as eyewitness accounts, in that they were penned by followers of Jesus who lived contemporaneously with him, such as from an apostle (John), or close associates of the apostles who recorded their first-hand accounts. However, there is much in the Gospels that could not be eyewitness accounts given the way the events themselves are described, if one uses the definition above.
Take, for example, meetings that the disciples could not have attended. In Matthew 26:3, verses 3-5, a meeting is described where the Jewish chief priests and elders secretly schemed to arrest and kill Jesus. The description includes a quote from the meeting. Who was the eyewitness to this meeting? Not Matthew or any of the disciples surely. So how did the details of this event reach Matthew’s ears? Similarly, a meeting between Judas and the priests is described in verses 14-16, where Judas is quoted negotiating as to his payment. Again, no disciples or their associates could have been present (other than Judas) – in fact, they were all shocked to learn that one of them would betray Jesus in verse 22. Had any follower of Jesus known of Judas’s impending betrayal, surely he or she would have warned Jesus and the disciples. So how did the details of the meeting between Judas and the priests reach Matthew—did Judas go up to the disciples after Jesus’s arrest to describe the meeting to them? That seems unlikely. Another problematic meeting is found in Matthew 27, verses 62-66. Here, the priests and Pharisees meet with Pontius Pilate himself, and scheme to put a guard on Jesus’s tomb. Quotes abound, suggesting that there was an “eyewitness” scribe there taking notes. But who could imagine Matthew or any other close associate of Jesus attending this meeting and jotting down quotes?
The Gospels also contain many scenes where an individual is alone or even dreaming when something miraculous happens, so none of the Gospel writers could have been eyewitnesses. In Matthew 20, verse 20, an angel appeared to Joseph in a dream. A dream isn’t an event that is perceived through one of the five senses, so Joseph is arguably not even an eyewitness here. How could Matthew be? In Luke 1 verse 11, an angel appears to Zechariah when he is by himself in the temple. In verse 28, the Angel Gabriel appears to Mary when she is alone. Mary and Zechariah can be considered eyewitnesses, but their testimony could not be confirmed by others, only passed along. Testimony that is passed along by word of mouth is heresay, not eyewitness testimony. So how could Matthew or Luke be described as eyewitnesses?