Obstacle on the radio path

An object or thing that causes an obstruction of radio wave propagation.

Offset

In a given transmission, this term refers to a change of the frequency of radiated signal in relation to its nominal frequency.

Omnidirectional antenna

Antenna that radiates uniformly in all directions in one plane (omnidirectional radiation pattern) with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis

Optical (visible) horizon

1) The locus of points at which direct rays (of waves) from an source of optical waves are tangential to the surface of the Earth. 2) The boundary of optics visibility.

Out-of-band emission

Emission on a frequency or frequencies immediately outside the necessary bandwidth which results from the modulation process, but excluding spurious emission.