Plain English Meaning
Airspeed isn't just one number. There are four different versions of it, each calculated a little differently. Some are read straight off the instrument, others are corrected for errors or for the thinness of the air at altitude. Pilots use the right one depending on what they're trying to figure out.
Full Definition
A category covering the four distinct airspeed values pilots work with: Indicated Airspeed (IAS), Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), and True Airspeed (TAS). Each represents the aircraft's speed through the air measured or corrected in a different way, accounting for , , air , and .