Copy editing (also known as copyediting and manuscript editing) is the process of revising written material (copy) to improve readability and fitness, as well as ensuring that a text is free of grammatical and factual errors. The Chicago Manual of Style states that manuscript editing encompasses "simple mechanical corrections (mechanical editing) through sentence-level interventions (line, or stylistic, editing) to substantial remedial work on literary style and clarity, disorganized passages, baggy prose, muddled tables and figures, and the like (substantive editing)". In the context of print publication, copy editing is done before typesetting and again before proofreading.^: 1–5 Outside traditional book and journal publishing, the term copy editing is used more broadly, and is sometimes referred to as proofreading, or the term copy editing sometimes includes additional tasks.

Although copy editors are generally expected to make simple revisions to smooth awkward passages, they do not have a license to rewrite a text line by line, nor do they prepare material on an author's behalf. Creating original content to be published under another person's name is called ghostwriting. Furthermore, copy editors are expected to query structural and organizational problems, but they are not expected to fix these problems. In addition, copy editors do not normally engage in developmental editing, which includes helping an author develop an idea into a publishable manuscript, overhauling a rough draft, identifying gaps in subject coverage, devising strategies for more-effective communication of content, and creating features to enhance the final product and make it more competitive in the marketplace.

In the United States and Canada, an editor who does this work is called a copy editor. An organization's highest-ranking copy editor, or the supervising editor of a group of copy editors, may be known as the copy chief, copy desk chief, or news editor. In the United Kingdom, the term copy editor is used, but in newspaper and magazine publishing, the term is subeditor (or sub-editor), commonly shortened to sub. In the context of the Internet, online copy refers to the textual content of web pages. Similar to print, online copy editing is the process of revising and preparing the raw or draft text of web pages for publication.

Copy editing has three levels: light, medium, and heavy. Depending on the budget and scheduling of the publication, the publisher will let the copy editor know what level of editing to employ. The chosen type of editing will help the copy editor prioritize their efforts.^: 12