An outline, also called a hierarchical outline, is a list arranged to show hierarchical relationships and is a type of tree structure. An outline is used to present the main points (in sentences) or topics (terms) of a given subject. Each item in an outline may be divided into additional sub-items. If an organizational level in an outline is to be sub-divided, it shall have at least two subcategories, although one subcategory is acceptable on the third and fourth levels, as advised by major style manuals in current use. An outline may be used as a drafting tool of a document, or as a summary of the content of a document or of the knowledge in an entire field. It is not to be confused with the general context of the term "outline", which a summary or overview of a subject, presented verbally or written in prose (for example, The Outline of History is not an outline of the type presented below). The outlines described in this article are lists, and come in several varieties.

A sentence outline is a tool for composing a document, such as an essay, a paper, a book, or even an encyclopedia. It is a list used to organize the facts or points to be covered, and their order of presentation, by section. Topic outlines list the subtopics of a subject, arranged in levels, and while they can be used to plan a composition, they are most often used as a summary, such as in the form of a table of contents or the topic list in a college course's syllabus.

Outlines are further differentiated by the index prefixing used, or lack thereof. Many outlines include a numerical or alphanumerical prefix preceding each entry in the outline, to provide a specific path for each item, to aid in referring to and discussing the entries listed. An alphanumerical outline uses alternating letters and numbers to identify entries. A decimal outline uses only numbers as prefixes. An outline without prefixes is called a "bare outline".

Specialized applications of outlines also exist. A reverse outline is a list of sentences or topics that is created from an existing work, as a revision tool; it may show the gaps in the document's coverage so that they may be filled, and may help in rearranging sentences or topics to improve the structure and flow of the work. An integrated outline is a composition tool for writing scholastic works, in which the sources, and the writer's notes from the sources, are integrated into the outline for ease of reference during the writing process.

A software program designed for processing outlines is called an outliner.