stagnate

Short definition from Wiktionary

Verb

1. 1
   To cease motion, activity, or progress: To cease to flow or run.
2. 2
   To cease motion, activity, or progress: To be or become foul from standing.
3. 3
   To cease motion, activity, or progress: To cease to develop, advance, or change; to become idle.

Adjective

1. 1
   Stagnant.