adjective
stagnant
STAG-nuhnt/ˈstæɡnənt/
Remaining motionless without circulation.
A film of algae covers the stagnant water in the pool.

Etymology
Latin stagnare described water standing without a current. Stagnant extends that image of stillness to things that are inactive, no longer developing, or failing to improve.
In context
Japan′s stagnant economy is expected to remain in the doldrums for another few years …
More examples
- Opening the windows helps circulate the stagnant air.
- Mosquitoes breed in stagnant water left in old tires.
- The fountain's pump keeps the water from becoming stagnant.
Forms
stagnant
stagnate verb — To stop flowing or circulating.
stagnation noun — A state without flow or circulation.