The natural state of a lifeform or ecosystem was the way it normally appeared in nature, without shape-shifting or terraforming.
The mellitus's natural state was gaseous. (TOS: "Wolf in the Fold")
Allasomorphs' natural state went beyond human imagination. (TNG: "The Dauphin")
In 2369, Data reported that the surface of Tagra IV had returned to its natural state. (TNG: "True Q")
Apart from weather modification, Risa was "a rain-soaked, geologically unstable jungle." (DS9: "Let He Who Is Without Sin...")
Changelings had to revert to their natural state every 18 hours to regenerate. (DS9: "A Man Alone") This state was typically experienced as part of the Great Link. According to Quark, this reversion was difficult for humanoids to grasp. (DS9: "Chimera")
In 2375, Weyoun suggested the Female Changeling revert to her natural state to rest. She admitted that she hadn't been able to change form for weeks at that point. (DS9: "What You Leave Behind")