Sentence:  The <WSD>matrix</WSD> describing the reflection at a plane mirror can be obtained by taking the <WSD>matrix</WSD> for reflection at a spherical reflector and letting the radius of the spherical mirror tend to infinity. 
Meaning of the word marked with <WSD> tags:  A rectangular arrangement of numbers or terms having various uses such as transforming coordinates in geometry, solving systems of linear equations in linear algebra and representing graphs in graph theory.