The ticket gate/barrier at a station where tickets or IC cards are checked; often simply called 改札 but 改札口 emphasizes the physical gate.
改札口で切符を見せてください。
Please show your ticket at the ticket gate.
新しい駅は改札口が三つある。
The new station has three ticket gates.
改札口の前で友達と待ち合わせた。
I met my friend in front of the ticket gate.