Well, it's not this one, as absolute speed does not exist. Speed is the scalar part of velocity which is a relative value. You cannot express speed without specifying speed relative to what.
People will try and say "Relative to a fixed point in space"- this ALSO does not exist.
Yes speed is relative to a point. For this, let's say speed is relative to a point on the surface of the earth. Makes things easy.
Now about the definition of groundspeed. It's just that!... the speed over the ground (surface of the earth). This is going to be true for navigation at any altitude.
Now looking at different altitudes, an aircraft traveling around the earth at 35,000' AGL will travel a further distance relative to the "same" point on the earth as an aircraft flying at 1000' AGL. Therefore, to travel over the surface of the earth at the same speed, the 35000' aircraft will have to travel faster! It will have a faster TAS. The TAS is how fast the aircraft is traveling through the air. Now to understand this concept we should state that we are disregarding density of the air, like TAS does.
In this little diagram I whipped up, the "groundspeed" is the same, but the TAS is not! The TAS would be the definition (2) that the OP wrote. The 1.0 and 1.42 are just rough estimates and nothing here was calculated, it was just made to clearly illustrate a point.
