Define the term "side out" in volleyball.

The term "side out" in volleyball refers specifically to the situation when the receiving team wins the rally and takes over the serve from the opposing team. This occurs after the receiving team successfully defended against the serve and played a rally, ultimately resulting in a point gained due to the failure of the serving team to win the rally. This rule emphasizes the competitive nature of volleyball, where teams strive not only to score points but also to regain the service advantage.

The other options do not accurately capture the definition of "side out." The act of serving being the first touch does not indicate any transition of serve. Hitting the ball out of bounds simply results in a point for the opposing team; it does not involve a change of service. Likewise, substituting a player does not relate to the concept of side outs, as it focuses on player rotations rather than the scoring or service change mechanics of the game.