Opposite side; the other side or the opposing side (spatially or in terms of stance/opinion).
反対側の席に誰か座っている。
Someone is sitting in the seat on the opposite side.
駅の反対側にスーパーがあります。
There's a supermarket on the opposite side of the station.
議論で彼は反対側の立場を取った。
In the debate, he took the opposing side.