Extended definition
Resources (CSS, JS) that block page rendering until they load. Solutions: async, defer, inline critical CSS.
Context and application
CSS implicitly is render-blocking — the browser waits to download and parse it before displaying anything. Solution: extract critical CSS (the one needed for above-the-fold) and inline it in <head>; the rest of the CSS loads async with media="print" onload trick or dynamic JS. Classic JavaScript in <head> is also blocking; async allows parallel download, defer postpones execution until after HTML parsing. type="module" has implicit defer. Tools like Critical or Penthouse generate critical CSS automatically.