The two documents do different jobs
In the United States, a resume is a short, targeted summary of the experience, skills, and education that matter for one job. A curriculum vitae (CV) is a longer, more complete record that traces your academic and professional history in detail, and it is usually read by people evaluating scholarly work.
That difference in purpose drives everything else. A resume is edited down to the evidence a specific employer needs, while a CV is expected to be comprehensive and can run many pages as your career grows. Treating the two as interchangeable usually leads to the wrong document in the wrong pile.
Let the role and region decide
The clearest signal is the kind of position. Academic roles, research posts, fellowships, and graduate programs commonly ask for a CV that includes education, publications, presentations, grants, and teaching or research experience. Most private- and public-sector jobs ask for a resume.
Location matters too. In the U.S., the two are distinct, but in the UK, New Zealand, and parts of Europe the word CV often describes what an American would call a resume. In Australia, India, and South Africa, the terms are frequently used interchangeably. When a posting uses one word, follow its local meaning.
Check the posting before you send anything
Read the application instructions closely: they usually name the document, the length, and sometimes the sections required. If the request is ambiguous, ask the recruiter or hiring manager which format they expect rather than guessing. It is a normal question, and it has a specific answer.
It can help to keep both versions on hand. A CV is essentially a more detailed version of a resume, so building one from the other is mostly a matter of restoring detail rather than starting over. The common mistake is sending a multi-page academic CV to a business role that wanted a concise resume.
Sources consulted
This guide is an original synthesis of the following career guidance. Read the source material for its full context.