Plain English Meaning
How hot or cold something is. In flying, it usually refers to how warm or cold the outside air is, because that affects how the airplane and engine perform.
Full Definition
A measure of the average kinetic energy of the molecules in a substance, expressed in degrees Fahrenheit (°F), Celsius (°C), or Kelvin (K). In aviation, temperature is one of the four primary variables (along with pressure, density, and humidity) used to describe the state of the atmosphere and to predict aircraft and engine performance.