A seedling was a young plant.
In 2270, Sarah April noted that her Capellan flower was a seedling that morning and would die within a few hours. (TAS: "The Counter-Clock Incident")
When Jean-Luc Picard, Guinan, Ro Laren, and Keiko O'Brien were transformed into children in 2369, Keiko's plants were also transformed into seedlings. (TNG: "Rascals")
In 2371, Kathryn Janeway noted mud and alfalfa seedlings stuck in a Ford truck found in space, surmising that its owner was either a farmer or at least lived in a rural area. (VOY: "The 37's")
In 2373, while attempting to assert her personality over Tieran's, Kes recalled that she felt most content watching seedlings grow in USS Voyager's airponics bay. (VOY: "Warlord")