The practical use of the term 'Moisture content' as it relates to concrete would probably be considered to mean the 'free water content', or water not expected to be chemically bound during the hydration process.
About .24 x the cement content of the concrete is used during hydration and the rest of the water (water of convenience) is not tied up and may move out of the cement.
The operative word here is 'may'. While concrete poured at a W/C ratio of .5 leaves us .26 of free water, that doesn't mean the water will actually leave the slab.
So if we have a slab with 400 pounds of cement and a W/C ratio of .5, we would have 200 pounds of water. Of this 200 pounds, .24 times the cement (.24 x 400 = 96 lbs) will typically be consumed or bound by a reaction of the cement. The remaining 104 pounds of water is not chemically bound and is free to move.
How much of this water will actually leave the cement during equilibration to ambient conditions will vary based upon many factors. Total equilibration can be expected to take many years, at which point a slab will eventually reach a steady state of water content. Many experts in the field say this steady state is at about 50% relative humidity, while others say it is near 75%. I believe the exact moisture content will vary based on external conditions, such as permeability of vapor retarder, temperature of the concrete, ambient RH levels, and the amount of time a slab takes to reach equilibrium will vary based on those same factors plus slab porosity, slab density, emission retarders (everything from sealers to surface debris), initial water content, amount of variance relative to surrounding ambient conditions, wind, elevation, rewetting, etc.
The magic indicator we crave is a method to tell us when a concrete slab has expired the minimum amount of its free moisture so that the flooring we install will not be affected by the remaining moisture retained. This indicator has changed over the years from a taped down piece of plastic to the current use of calcium chloride crystals and electronic probes sampling air at defined isolated depths. Which camp you subscribe to probably will depend on which method the adhesive and flooring manufacturer propose.
Being aware of the limitations of our test methods and that we are at the mercy of external sources of moisture not readily discernible by our testing is important to note. We must also be aware of changing hypotheses of the effects of moisture on alkalinity and adhesives as the industry works through these problems. Remember it was only a short time ago that many subscribed to the thought that a low MVER was a guarantee that concrete moisture was acceptable for sensitive flooring installation.
So to the original question of 'What is moisture content': Technically it is the amount of moisture in the slab. Realistically it is the amount of free moisture we will be expected to be deal with. In that context we need to consider all the factors of free moisture and predict how much free moisture is acceptable. To that end we need 3 things; a method to measure the moisture, a limit to measure to, and verification that additional moisture is not going to be introduced.
I believe RH testing is our best indicator of the free moisture in concrete (especially when coupled with other common sense observations and tests), many manufacturers are now publishing RH guidelines for their products and we can all do our best to verify additional water is not getting into our concrete.
Sorry for the long windedness.....
Again, I'm no genius so please point out errors, omissions and mistakes. I learn something new every day!