From SKYbrary Wiki
The temperature of the air outside an aircraft is measured and indicated within the cockpit or used, together with outputs from the Pitot Static System, as an input to aircraft equipment, e.g. Air Data Computer (ADC).
Outside Air Temperature (OAT)
The ambient temperature measured outside an aircraft is known as the Outside Air Temperature (OAT) or Static Air Temperature (SAT). The sensor which detects OAT must be carefully sited to ensure that airflow over it does not affect the indicated temperature.
Total Air Temperature (TAT)
If temperature is measured by means of a sensor positioned in the airflow, kinetic heating will result, raising the temperature measured above the OAT. The temperature measured in this way is known as the Total Air Temperature (TAT) and is used in ADCs to calculate True Airspeed (TAS). Careful design and siting of the TAT probe is necessary to ensure accurate measurement of TAT.