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

See also: unofficial

Antonym: officially

• syncopation
• syncopator
• syncope
• syncretic
• syncretical
• syncretise
• syncretism
• syncretist
• syncretistic
• syncretistical
• syncretize
• syncytium
• syndactylism
• syndactyly
• syndesmosis
• syndetic
• syndic
• syndical
• syndicalism
• syndicalist
• syndicate
• syndication
• syndicator
• syndrome
• syne