noun which may take the genitive case particle 'no'
unauthorized; without permission; unapproved
Used as a no-adjective to describe actions or things done without official permission.
You must not start construction without permission.
Unauthorized copying is a violation of the law.
lack of permission; absence of authorization
Used as a noun to refer to the state of not having permission. Often appears in formal or legal contexts.
The lack of permission became an issue.
The contractor was carrying out the construction without a permit.
Unauthorized parking is subject to a fine.
They entered the factory grounds without permission.
無断 is a more casual, everyday word for acting without asking (e.g. 無断欠席), while 無許可 stresses lacking formal, official authorization.
無許可 stresses acting without obtaining official permission/license, whereas 無届け stresses failing to file the required notification, which may or may not require approval.
Compound of 無 (mu, 'without') and 許可 (kyoka, 'permission').