The term Hitbox refers to the area of influence of the player in a shmup game. If any object touches it, then the player is affected in some way. If this object happens to be a bullet or wall, the player is usually considered dead or damaged.
In most early shmup games, the hitbox was about the size of the player sprite. In most modern shmups, the hitbox is about 1X1 pixels in size, allowing the player to squeeze through rows of very close bullets.
In shmups involving shields, the player's hitbox extends to the size of the shield until it runs out (if possible).