A Federation starbase where Picard attended an emergency conference in 2364 after contact was lost with outposts along the Romulan Neutral Zone.
Starbase 718 is a Starfleet facility mentioned in the first-season Next Generation finale The Neutral Zone. It is never seen on screen, but events there drive the episode.
In 2364 Captain Picard is summoned to an emergency conference on the starbase and travels there by shuttle, leaving the Enterprise-D waiting for him. While he is away Data and Worf explore a derelict twentieth-century Earth capsule and bring three cryogenically frozen humans aboard.
Picard returns with grim news. Two Federation outposts on the edge of the Neutral Zone have gone silent, and Starfleet suspects the Romulans, who have not been heard from in decades. The Enterprise is ordered to investigate, leading to the first face-to-face contact with the Romulans in more than fifty years.