quadrature

1. In astronomy, the arrangement of the earth, sun, and another planet or the moon in which the angle subtended at the earth between the sun and the third body, in the plane of the ecliptic, is 90°.
   The first and third quarters of the moon are positions of quadrature.
   See also conjunction, opposition.
   
2. In radar systems, an orthogonal relationship between two coherent signals in which the phase of one signal is offset by 90° from the phase of the other. Two signals in quadrature may be regarded as a single complex signal.