1. Indicated: IAS
Indicated airspeed is the speed that the plane “feels”. It might help to think of it as the number of air molecules hitting the plane. This is the speed that matters for the performance of the plane. It can be read directly on the airspeed indicator.
2. True: TAS
As you climb the air gets thinner. As the air gets thinner there are fewer air molecules in a given volume of air. This allows the aircraft to fly faster.
For example, if your plane has enough power to fly at 100 knots and you maintain 100 knots while climbing your true airspeed will increase. True airspeed is your actual speed through the mass of air. As you climb and the air thins out, if you are still at 100 knots then you are still encountering the same amount of air over time, but since that air is spread out over a longer distance you are flying at a faster speed. True airspeed is the same as groundspeed if there is absolutely no wind.
This bonus in speed and better fuel economy are the reasons that planes bother to climb all the way up to high altitudes.
3. Calibrated: CAS
Airspeed indicators aren’t perfect. When flaps are down or the plane is at a high angle of attack the airspeed indicator may be off by several knots. This error is studied and a placard is provided with the correct numbers. So calibrated airspeed is more precise than indicated airspeed but it is not displayed directly on the airspeed indicator.
4. Ground Speed: GS
This is not an airspeed, but it is worth including here. Ground speed is the speed that really matters for getting somewhere, it is very simply your speed over the ground. It is equal to your true airspeed plus or minus a tailwind or headwind.
When there is enough wind it is possible to gain an enormous amount of extra speed. This is why jets love to take advantage of the jet stream where the wind speed can often exceed 100 knots.
It is also possible to make a plane stop or fly backward. See the video below that illustrates this concept.
5. Equivalent Airspeed: EAS
Equivalent airspeed is a further correction of calibrated airspeed that corrects for airspeed indicator errors due to compressibility. It is most prominent at high altitudes and high speeds. Modern planes that can reach these altitudes and speeds generally have an air data computer that handles the calculation of EAS but a simple performance chart can be used as well. For light aircraft, EAS is generally ignored because it is very close to being the same as CAS.
6. Mach Number: M
Aircraft that fly at higher altitudes and speeds, like jets, generally refer to their speed in terms of mach number. This speed is measured as the ratio of the speed of sound. For example, mach 1 means you are flying at the speed of sound, and mach .5 means you are flying at half the speed of sound.
Pilots generally pronounce mach numbers like “mach point seven five”, or “mach point eight” for M.75 and M.8, respectively.
Most planes fly at subsonic speeds, less than the speed of sound.
Faster planes like the Concorde, and some military fighters and bombers can fly at transonic speeds, at the speed of sound. They can then accelerate to supersonic speeds great than the speed of sound.
Anything greater than M5.0 is considered to be a hypersonic speed. Hypersonic aircraft are certainly being studied but as far as I know, there aren’t any flying.
As an aircraft reaches higher altitudes the mach number is used to measure speed instead of IAS. An aircraft is limited in IAS by aerodynamic pressure and in mach number it is limited by aerodynamic shock waves. Since there are two different limits they both need to be considered.
For example, a Boeing 757 has a Vmo (maximum operating airspeed) of 350 knots and an Mmo (maximum mach number) of M0.86.
At a low altitude near sea level M.86 is 568 knots, well beyond the Vmo limit. However, at a high altitude like 40,000 feet M.86 is approximately 262 knots, well below the Vmo limit. This is why pilots will reference IAS at low altitudes and mach number at high altitudes. During climb there is a crossover altitude at which the transition is made from thinking in IAS to thinking in mach.