What must a player's serve clear in order to be considered valid?

For a serve to be considered valid in volleyball, it must clear the net. This is fundamental to the rules of the game, as the ball must travel over the net and into the opponent's court to initiate play. If the serve does not clear the net, it results in a fault, stopping play immediately and giving a point to the opposing team.

While the center line, players on the opposing team, and the service line are all relevant aspects of the game, they do not determine the validity of a serve. The center line is used to define the boundaries between the two sides of the court, and the service line indicates where the server must stand when serving. However, neither of these lines is essential for a serve to be valid; the key requirement is that the ball must go over the net unimpeded.