Half-space may refer to: Half-space (geometry), either of the two parts into which a plane divides Euclidean space (Poincaré) Half-space model, a model of hyperbolic geometry using a Euclidean half-space Siegel upper half-space, a set of complex matrices with positive definite imaginary part Half-space (punctuation), a spacing character half the width of a regular space Half-space model (oceanography), an estimate for seabed height in areas without significant subduction