A reserved seat that is assigned in advance (used for trains, theaters, concerts, etc.). The opposite is 自由席 (non-reserved seat).
今回は指定席を予約したので座席は確保されている。
I reserved a seat this time, so my seat is secured.
指定席が満席なら自由席で立って行くしかない。
If reserved seats are full, the only option is to take a non-reserved seat and stand.
指定席券は駅の窓口かオンラインで購入できる。
Reserved-seat tickets can be purchased at the station counter or online.
コンサートの指定席はグループごとに近い席が取りやすい。
Concert reserved seats often make it easier for a group to get seats close to each other.