Definition - What does Competent Witness mean?
A competent witness is a witness who can be called to the stand due to being legally qualified. Qualification criteria include mental fitness, age, proper understanding of the issue at hand, etc. Essentially, a competent witness is a witness that the court has deemed acceptable to give a testimony in a court case.
Justipedia explains Competent Witness
In trials, there could be problems if witnesses are not competent. For example, if a two-year-old was allowed to give a testimony, there is a high-percent chance that he or she would not fully understand the matter, nor be able to give an accurate testimony. Severely mentally ill people, or people with brain damage, could also have trouble giving credible testimonies. Due to the possible credibility issues, the court only allows witnesses that it deems competent to testify.