An intensely private species who knocked out the Enterprise-D crew and, with Data's help, removed a full day from their memories to keep their existence secret.
The Paxans are a xenophobic species who live within a protected region of space and are determined never to be discovered. Their territory is hidden by a trap that can render a starship's crew unconscious, and they are prepared to destroy any visitors who learn of them.
In the episode Clues, the Enterprise-D crew woke from what seemed a brief blackout, but small inconsistencies made them realise far more time had passed. Data had been ordered by Captain Picard to hide the truth: the Paxans had threatened to destroy the ship, and Picard had agreed that Data would erase the crew's memories. When the first attempt failed, the crew had to repeat the cover-up more carefully.