Definition
Twitter Card is X (formerly Twitter)'s meta tag standard for rich-media share cards. Card types: summary, summary_large_image, app, player. Tags: twitter:card, twitter:site (@handle), twitter:creator, twitter:title, twitter:description, twitter:image. X uses its own tags first, falls back to Open Graph if missing. The summary_large_image type produces a full-width preview, with significantly higher engagement than the default small card.
<meta name='twitter:card' content='summary_large_image' /> = full-width preview. Without it, X shows a small card with thumbnail.
How to optimize
Set twitter:card to summary_large_image. Set twitter:site to your brand @handle. Reuse og:image as twitter:image. Validate with X's Card Validator.