the eighth part of a circle; an arc of 45 degrees.-the position or aspect of a heavenly body, as the moon or a planet, when half way between conjunction, or opposition, and quadrature, or distant from another body 45 degrees.-an instrument for measuring angles (generally called a quadrant), having an arc which measures up to 9o¡, but being itself the eighth part of a circle.  cf. sextant.-one of the eight parts into which a space is divided by three coordinate planes.