1. Without official authorization
   - on the side 
2. Not in an official capacity
   "unofficially, he serves as the treasurer"

See also: unofficial

Antonym: officially

• incentivize
• inception
• inceptive
• incertain
• incertitude
• incessancy
• incessant
• incessantly
• incessantness
• incest
• incestuous
• incestuously
• inch
• Incheon
• inchmeal
• inchoate
• inchoative
• inchoative aspect
• Inchon
• inchworm
• incide
• incidence
• incidence angle
• incident
• incidental