Z-AXIS - In a gyro, an axis through the center of gravity and mutually perpendicular to both the X (spin) and Y axes .

ZENER DIODE - A PN-junction diode designed to operate in the reverse-bias breakdown region .

ZENER EFFECT - A reverse breakdown effect in diodes in which breakdown occurs at reverse voltages below 5 volts. The presence of a high energy field at the junction of a semiconductor produces the breakdown .

ZEROING - The process of adjusting a synchro to its electrical zero position .

ZERO-MODULATION NOISE - (See noise.)

ZONE OF MUTUAL VISIBILITY - The area where the satellite can be seen by both the up- and down-link earth terminals .