voljno

1. voluntarily; willingly
   Niko ga nije terao, voljno je pristao. — Nobody forced him; he agreed willingly.

2. (military command) at ease
   Oficir je viknuo: „Voljno!“ i vojnici su se opustili. — The officer shouted "At ease!" and the soldiers relaxed.

In the sense "voluntarily", dobrovoljno is more common.