Secondary evidence is defined as a copy of evidence or oral description of its contents. It is considered not as reliable as best evidence. Evidence that proves or disproves a specific act through oral testimony based on information gathered through he witness’s five senses is considered direct evidence. The fact that testimony is given by an expert only affects the witness’s ability to offer an opinion instead of only testifying of the facts.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 9: Law, Investigation, and Ethics (page 310).