In everyday parlance, a contemporary means someone who lived at the same time as another person.
In conversations with mythicists, Paul is dismissed as not a contemporary of Jesus, because he did not (as far as we know) see him while he was alive.
But this brings into focus one of mythicism’s many problematic aspects. Mythicists often insist that Jesus was a significant figure in his time, so that we ought to expect historians to have mentioned him and satirists to have poked fun at him.
But when someone is that well known in a person’s time, they are well poised to know of their existence even if they have never met them personally.And so if Jesus was a widely-known figure, then Paul’s testimony is adequate. And if Jesus was not so well-known, then the lack of testimony from other contemporary sources would be unsurprising, even if we had an abundance of sources from Jesus’ time and location, which of course we do not.