Noun
horosphere (plural horospheres)
An
n
−
1
{\displaystyle n-1}
-dimensional hyperplane in hyperbolic
n
{\displaystyle n}
-dimensional space: it is (in the Poincaré disk) Euclidean-tangent at infinity to the boundary sphere or (in the upper-half-space model) Euclidean-parallel to the boundary hyperplane.