(covers information from several alternate timelines)
Ping pong, or table tennis, was an Earth sport in which two or four players hit a small ball back and forth quickly to each other on a smooth table. It was considered an excellent test of hand-eye coordination.
The USS Voyager held a ping pong tournament in 2375. Tom Paris invited Seven of Nine to be his doubles partner after William Chapman came down with an apparent case of space sickness. They played against B'Elanna Torres and Harry Kim.
In an alternate timeline, the ping pong tournament was interrupted when a temporal disruptor destroyed Voyager. The USS Relativity took Seven of Nine aboard to avert the disaster, which produced multiple alternate timelines, including one where she interacted with herself during the ping pong tournament. These incursions were eliminated after Captain Braxton was determined to be the perpetrator. (VOY: "Relativity")