special issue

Short definition from Wiktionary

Noun

1. 1
   An issue taken by denying a particular part of the allegations.
2. 2
   An issue that is different from the usual run of issues, usually because it is a theme issue: one in which the articles are unified by a theme. Some supplements are of this character, and whether such an issue is called a supplement or a special issue is determined by each journal's style.

Adjective

1. 1
   Not of the usual type issued by an organization.