Jira Wiki Markup is Atlassian's legacy formatting syntax for Jira fields that use the Wiki Style Renderer. It is not Markdown; it uses Jira-specific symbols and macros such as
h2.,{code},{panel},||table headers||, and!image.png!. Use it when a Jira description, comment, custom field, or admin/help screen expects raw wiki markup. If a modern Jira editor does not render this syntax, the field likely is not using the wiki renderer.
Original source: Atlassian Jira Wiki Renderer Help