Matthew Picard

From Memory Alpha, the free Star Trek reference.

Matthew Picard was Jean-Luc Picard's son in the nexus, but never really existed. Captain Picard encountered him at a Christmas celebration that he imagined, as having children and carrying on the Picard family line was one of his deepest wishes, but he decided to return to "real life," in 2371. (Star Trek Generations)

Matthew Picard was played by young actor Matthew Collins, whose real-life sister Mimi Collins also played a Picard child, Mimi Picard.