Redline

Redline is a term with several related meanings in English. It can refer to a physical line drawn in red on maps, documents, or other surfaces, used to mark a boundary, limit, or mark of changes. In editing and document management, a redline version shows proposed changes by highlighting deletions and insertions in red, allowing reviewers to compare versions and approve edits. The verb "to redline" is used to describe the act of marking or reviewing edits in red ink or red text.

The most well-known social use is redlining, a discriminatory practice in lending and insurance where services are denied or limited in neighborhoods based on race or ethnicity rather than risk. In the United States, redlining was institutionalized in the mid-20th century through lender maps and underwriting practices, contributing to residential segregation and wealth inequality. Legislation such as the Fair Housing Act of 1968 sought to curb it, but effects and related practices persist in various forms.

Red line or redline is also used metaphorically to describe a non-negotiable threshold in policy, diplomacy, or operations. Crossing a red line is presented as triggering a decisive response.

In modern usage, Redline appears as a proper name for products, brands, or media titles, including automotive parts, energy products, and entertainment works. The term’s versatility reflects its visual clarity as a marker of boundaries, changes, and critical limits.