A people with whom Picard is holding a conference when one of their factions opens fire, striking him in the chest and fusing his artificial heart at the start of Tapestry.
The Lenarians are a species involved in talks with the Federation in 2369. They are not seen clearly on screen, and all that is said of them is that at least one group among them is hostile enough to attack a Starfleet delegation with energy weapons.
They are mentioned in the Next Generation episode Tapestry. Picard is beamed straight to sickbay after an away team is ambushed at a Lenarian conference. A compressed teryon beam has hit him in the chest and damaged his bio-regulator, and Doctor Crusher is unable to keep his artificial heart working.
Picard then finds himself with Q, who tells him he is dead and offers the chance to undo the bar fight with a Nausicaan that cost him his real heart. He wakes in sickbay at the end, alive and laughing as he did as an ensign.