A field of energy surrounding the rim of the Milky Way that damages ships attempting to cross it and can awaken dangerous psychic powers in sensitive members of a crew.
The galactic barrier is first met in Where No Man Has Gone Before, set in 2265. The Enterprise tries to pass through as the lost SS Valiant once did; the ship is crippled, several crew die, and Gary Mitchell and Elizabeth Dehner begin to develop godlike abilities.
The Enterprise crosses it again in By Any Other Name, when Kelvan agents seize the ship for a voyage to Andromeda, and in Is There in Truth No Beauty?, when a maddened Larry Marvick drives the ship beyond it.
Discovery returns to the barrier in its fourth season. In The Galactic Barrier and the episodes that follow, the ship passes through to reach Species 10-C, the makers of the Dark Matter Anomaly.