What counts as an 'ultralight' backpack?

Reviewers generally use the term for backpacking packs under about 2 pounds (roughly 900g), often frameless or with a minimal removable frame, and frequently built in Dyneema Composite Fabric (DCF) or robocoated nylons. Framed packs like the Osprey Exos 38 sit on the heavier edge of the category but are still widely classed as ultralight by mainstream outdoor publishers.