Borrowed from the mid-century art and architecture movement; NN/g's 112-site study pinned down what the word means on the web specifically.

What makes this style what it is.
Minimalism governs what is on the screen, not how it's drawn: strip the interface to the fewest elements that still do the job, then let generous negative space and one or two dramatic typographic moments carry the composition. NN/g's study of 112 minimalist sites found the recurring traits — flat-ish surfaces, limited or monochrome palettes, restricted element counts, negative space, and large type as the hero.
Build the UI using Minimalism. Strip the interface to the fewest elements that still do the job. Negative space is material — emptiness is deliberate and generous; the space around a thing is what makes it matter. Restricted element count: few things on screen, every one earns its place — one nav, one message, one action. Limited or monochrome palette: one or two hues at most, often just black/white/grey with a single accent. Dramatic typography: with imagery and ornament gone, type goes big — an oversized headline is the loudest element on the page. CSS starting points: - Layout: max-width: 720px; margin: 0 auto; padding: 120px 24px; - Background: #fff or #fafafa; text: #111; accent: one color only - Headline: font-size: clamp(3rem, 8vw, 6rem); font-weight: 700; letter-spacing: -0.03em; line-height: 1.05; - Body: font-size: 1.125rem; line-height: 1.6; color: #555; max-width: 55ch; - Buttons: border: none; background: #111; color: #fff; border-radius: 6px; padding: 14px 32px; font-size: 1rem; - Remove everything that doesn't earn its place Do not drift into Flat Design: minimalism is about content reduction, not just unshaded 2D rendering. A minimalist page can have shadows or gradients if they serve the composition. Do not drift into Web Brutalism: the sparseness is polished and art-directed, not raw browser defaults. Preserve 4.5:1 text contrast, provide visible focus rings, and ensure the single accent color meets contrast requirements.
The reduction ethos of mid-century movements (Bauhaus, Swiss typography, 'less is more') applied to interfaces; on the web it became the dominant 'serious brand' register through the 2010s, with Apple's product pages as the most-copied specimen. NN/g's definitional study is from 2015.
This is minimalism because the content itself is reduced — few elements, vast negative space, one dramatic headline — whatever the rendering. It would be merely Flat Design if the screen filled up with content again and only the unshaded 2D rendering remained. Vs Web Brutalism: this is minimalism because the sparseness is polished and art-directed — a considered composition of type, space, and one action.
Explore connected design movements.