HDR High Dynamic Range
Images and video with a wider span between darkest and brightest point than traditional SDR, plus more colours. Requires 10 bits per channel and a display that can actually get bright.
The key is the transfer function: SDR uses a gamma curve defined relative to a 100-nit studio monitor, while HDR uses PQ (SMPTE ST 2084) where each value maps to an absolute brightness up to 10,000 nits. Formats: HDR10 (open, static metadata, the baseline), HDR10+ and Dolby Vision (dynamic per-scene metadata), HLG (backward compatible, for broadcast). The colour space is usually Rec. 2020 or DCI-P3. Warning: many cheap displays accept an HDR signal without being able to reproduce it — below roughly 600 nits and without local dimming the result often looks worse than SDR.