Structured data
Machine-readable markup, usually schema.org in JSON-LD, describing what a page is about and making it eligible for rich results.
Structured data labels a page's content — a product's price, a recipe's cooking time, an article's author — so search engines can understand it and display rich results. Google documents which types it supports and their required properties.
Markup must describe content visible on the page. Google has retired several rich result types over the years, so eligibility is worth checking against its current documentation.
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