Outliners

An outliner is a software tool or conceptual approach that organizes information in a hierarchical outline. It represents content as nodes arranged in levels, with parent nodes containing child topics. The interface typically supports indentation, collapsing and expanding sections, and quick navigation, enabling users to structure ideas, notes, or plans while preserving an overview.

Outliners provide multi-level trees, drag-and-drop reordering, and keyboard-driven navigation. Many offer rich text formatting, checklists, hyperlinks, and attachments. Some integrate with calendars, tasks, or databases, allowing planning and progress tracking. Different products emphasize either pure hierarchies (outline processors) or broader document-or knowledge-management capabilities (tree editors or outlining features in general-purpose apps).

Originating in the 1980s on personal computers, outlining tools evolved from simple hierarchical notes to feature-rich applications. Over time they have become common in writing, research, project planning, and knowledge management. Contemporary tools often blend outlining with note-taking, word processing, and collaboration, supporting cross-linking, tagging, and cloud synchronization.

Outline data are commonly stored in proprietary formats, but OPML (Outline Processor Markup Language) provides a widely adopted standard for exchanging outlines between applications. Outlines can usually be exported as plain text, HTML, or PDF, and many tools offer device synchronization and collaboration features to support team workflows.