concavity

dent

Shallow surface depression

• dimpleFacial skin depression
• indentationInward cut or setback
• depressionProlonged low state

hollow · pockmark · divot · impression · gouge · pit · crater · nick · groove · furrow

cavity

Empty space within

• socketHollow receiving cavity
• fossaAnatomical depression or hollow
• orbitCurved celestial path

sinus · glenoid · navel · dimple · depression · recess · ventricle · atrium · alveolus · fundus

crater

Impact or volcanic basin

• basinConcave water receptacle
• valleyLow ground between heights
• depressionProlonged low state

sinkhole · hollow · dell · gorge · canyon · caldera · cirque · ravine · gully · glen

alcove

Recessed area in room

• nicheRecessed wall space
• recessPause in activity
• bayPartially enclosed water

apse · grotto · vault · cove · inglenook · oriel · embrasure · arcade · loggia · portico

arc

Curved line segment

• camberDesigned surface curvature
• crescentCurved sickle-like arc
• parabolaSymmetrical open plane curve

hyperbola · catenary · meniscus · scallop · ogee · cove · flute · channel · trough · dish

cave

Underground hollow in rock

• cavernLarge natural underground chamber
• grottoSmall scenic cavern
• abyssImmeasurably profound depth

chasm · void · crevasse · fissure · rift · trench · pit · shaft · pothole · lava tube

• geometry termscircle, line, rectangle, side, square
• gapsgap, rift, void, chasm, crack
• hollowscavity, crater, hollow, dent, hole
• propertiessize, shape, color, height, width
• polygon featuressides, vertices, edges, angles, perimeter
• slope featuressteepness, gradient, grade, pitch, angle