Extended definition
Tag telling Google a page has versions in different languages (RO/EN/DE/...). Critical for international sites.
Context and application
Hreflang implementation is one of the most prone-to-error SEO tactics. Three implementation modes: 1) <link rel="alternate" hreflang="..."> tag in head (most common), 2) HTTP header (for PDFs), 3) Sitemap (rarer). Critical rules: ALL language versions must reference each other (bidirectional), include self-reference (RO → RO + RO → EN), use ISO 639-1 format (en, not eng) + optional country (en-GB vs en-US). Common mistakes: hreflang missing on variants, asymmetric reference, wrong format. Validation in Google Search Console “International Targeting”.
In practice
our site has RO as default and EN with hreflang en for international audience.