What is the meaning of the percentage used to describe the thinness of this glider's airfoil? In a glider review for the Schempp-Hirth Discus, I encountered the following paragraph:


  The Discus 2 airfoil is thin (about 14.5 percent), incorporating studies by K.H. Horstmann and Dr. Wuerz (wing) and Luc Boermans (tail). 


After researching this a bit, I've been unable to find an explanation for what "14.5 percent" was referring to.

I thought perhaps that it is the ratio (14.5/100) of the thickness of the wing to the width of the wing, but that is really just an educated guess.

The Wikipedia page on airfoils has a section entitled Thin airfoil theory, with two bullets which also might qualify:


  (1) on a symmetric airfoil, the center of pressure and aerodynamic center lies exactly one quarter of the chord behind the leading edge
  (2) on a cambered airfoil, the aerodynamic center lies exactly one quarter of the chord behind the leading edge


Is this "one quarter" the ratio that is mentioned in the article?

I did see this post, but had trouble seeing how the 90% and 99% numbers mentioned in the answer are related.
14.5% given in the article is the (maximum) thickness to chord ratio, expressed in percentage. It is usually used to explain how thin (or thick) the airfoil is. Note that this is a characteristic of the airfoil.
From the NASA page Wing geometry definitions:

Airfoil thickness to chord ratio, image from NASA page Wing geometry definitions

... The straight line drawn from the leading to trailing edges of the airfoil is called the chord line. ... The maximum distance between the two lines is called the camber, which is a measure of the curvature of the airfoil (high camber means high curvature).
The maximum distance between the upper and lower surfaces is called the thickness. Often you will see these values divided by the chord length to produce a non-dimensional or &quot;percent&quot; type of number.