Defining what is a “witness” & describing a “primary source”

2 Cor. 13:1, says the criteria for verification of a thing is “2 or 3 witnesses.”  The word “witness” is marturion (the word from which we get the word “martyr”–which was a courtroom witness).  The legal term used today is “primary source.”  Not a “secondary source:”, but a “primary” one.  What does that mean?