A segment is a short, independently downloadable piece of the video timeline - typically 2-6 seconds - and the fundamental unit of HTTP adaptive streaming. The player reads the manifest, then fetches segments one after another, deciding at each boundary which rendition's segment to pull.
Segment duration is a core tuning knob. Shorter segments lower latency and make switching more responsive but increase request overhead and can hurt cache and compression efficiency; longer segments do the reverse. Because segments are plain HTTP objects, they cache beautifully in any CDN, which is what makes internet-scale delivery cheap.
