Glossary
Every metric and signal a Lightscore report can show you, defined in one line each. Terms with a full page go deeper on how the metric is measured and what actually moves it.
Core Web Vitals & performance
- Largest Contentful Paint (LCP) Time until the largest thing in view (hero image, headline) has painted.
- Interaction to Next Paint (INP) How long the page takes to visibly respond to a tap, click or key press.
- Cumulative Layout Shift (CLS) How much the page jumps around as it loads. Lower is better; 0 is rock-steady.
- Total Blocking Time (TBT) How long the main thread was busy and couldn’t respond to taps or clicks.
- First Contentful Paint (FCP) Time until the browser paints the first text or image.
- Speed Index How quickly the page’s visible area fills in, on average.
- Time to Interactive (TTI) Time until the page reliably responds to input.
- Core Web Vitals Google’s three headline UX metrics: LCP (load), INP (responsiveness), CLS (stability).
Delivery & response
- Time to First Byte (TTFB) How long the browser waited before the first byte of the response arrived.
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Content Delivery Network (CDN) A network of edge servers that serve your content from near the visitor.
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HTTP version The HTTP protocol the origin negotiated with us — HTTP/1.1 or HTTP/2.
TLS & security
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TLS certificate The certificate that proves the site’s identity and encrypts the connection.
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Security headers grade An A–F grade for the response’s security headers, à la securityheaders.com.
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HTTP Strict Transport Security (HSTS) Tells browsers to only ever reach this site over HTTPS.
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Content Security Policy (CSP) A header that whitelists where scripts, styles and other resources may load from.
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Mixed content Insecure http:// resources loaded by an otherwise-https page.
Caching
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Cache-Control The header that tells browsers and CDNs whether and how long to reuse a response.
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Cacheable Whether this response is allowed to be stored and reused.
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ETag A fingerprint that lets the browser revalidate cheaply ("still the same?").
SEO & crawlability
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robots.txt The file at /robots.txt that tells crawlers what they may fetch.
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Sitemap An XML list of a site’s URLs to help search engines discover pages.
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Canonical URL The "official" URL for a page, when the same content has several addresses.
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Meta robots A per-page directive (index/noindex, follow/nofollow) for search engines.
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hreflang Markup that maps a page to its translations / regional variants.
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structured data (JSON-LD) Machine-readable schema.org data describing the page (Article, Product, …).
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Open Graph Meta tags that control how the page looks when shared (title, image, description).
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Viewport The meta tag that makes a page render at device width on mobile.
Diagnostics & page weight
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Third party Code and assets loaded from another origin — analytics, fonts, ads, widgets.
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Render-blocking resources CSS/JS in the <head> that the browser must fetch before it can paint.
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Main-thread work Total time the browser’s single UI thread spent parsing, compiling and running code.
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DOM size How many elements (nodes) the page renders.