(1):

(v. t.) 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.

(2):

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

(3):

(a.) Stagnant.