To cease to flow; to be motionless; as, blood stagnates in the veins of an animal; hence, to become impure or foul by want of motion; as, air stagnates in a close room.

To cease to be brisk or active; to become dull or inactive; as, commerce stagnates; business stagnates.

not flowing, dull, inactive

Stagnant.

be idle; exist in a changeless situation; "The old man sat and stagnated on his porch"; "He slugged in bed all morning"

When something stops improving. Also used when a plan is not developing anymore.

cause to stagnate; "There are marshes that stagnate the waters"

stand still; "Industry will stagnate if we do not stimulate our economy"