Symbolic annular region; stays unevaluated and is consumed by region functions (RegionDimension/RegionMeasure/…)
Annulus[{x, y}, {r inner, r outer}]
Annulus[{x, y}, {r inner, r outer}, {θ 1, θ 2}]
Graphics[Annulus[]]
→ -Graphics-Area[Annulus[{0, 0}, {1 / 2, 1}]]
→ (3*Pi)/4Area[Annulus[{0, 0}, {r / 2, r}, {0, 3Pi / 2}]]
→ (9*Pi*r^2)/16