Rebuffering (buffering, stalling) is when playback stops to refill the buffer because segments aren't arriving fast enough — the spinning loader mid-video. It is the single QoE problem most strongly tied to viewer abandonment and, for subscription services, to churn: viewers tolerate lower resolution far better than they tolerate a stall.
Rebuffering is usually measured two ways: the rebuffer ratio (time spent stalled as a share of total watch time) and rebuffers per hour (frequency). Both matter — one long stall and many short ones feel different. The causes span the whole stack: insufficient network throughput, an ABR algorithm that ramps too aggressively, CDN or origin problems, an encoding ladder with no low-enough rung, or a too-small player buffer.
Because it is so consequential, reducing rebuffering drives a lot of engineering: better ABR tuning, a well-designed ladder with a reliable bottom rung, multi-CDN steering away from degraded paths, and low-latency techniques balanced against the buffer headroom they sacrifice.
