1. Being in effect or operation
   "bus service is in operation during the emergency"
   - operational, operating

See also: operative

• arccos
• arccosecant
• arccosine
• arccotangent
• arcdegree
• arced
• arcella
• Arcellidae
• Arceuthobium
• Arceuthobium pusillum
• arch
• arch over
• arch support
• archaean
• archaebacteria
• archaebacterium
• archaeo-
• archaeobacteria
• archaeologic
• archaeological
• archaeologically
• archaeologist
• archaeology
• archaeopteryx
• Archaeopteryx lithographica