Ten Centimetre Flux
The power output of the Sun at a wavelength of 10.7 centimetres, frequently used in place of sunspot number as an indicator of solar activity.

Terminal
In a HF prediction, this is the location of one of the two transceivers, e.g., Sydney.

TID
See Travelling Ionospheric Disturbance.

T Index
An indicator of the effect of solar activity on HF frequencies. The T index is an indicator of the highest frequencies able to be refracted from regions in the ionosphere. The higher the T index, the higher the frequencies able to be refracted from an ionospheric region.

Total Electron Content
The number of electrons along a wave path measured in electrons/square cm. The TEC is used in determining delay and directional changes of a wave in the ionosphere.

Travelling Ionospheric Disturbance
These are wave-like motions in the ionosphere that cause focusing and defocusing of radio waves. The disturbance usually appears at higher frequencies, gradually affecting lower frequencies. Some are associated with magnetic storms and originate in the auroral zone. These may travel great distances. Others appear to be more localised, originating in the troposphere in local weather.

Troposphere
The lowest layer of the Earth's atmosphere extending up to about 13 km in altitude.