Sentence:  The efforts and accomplishments fall into three main categories : absolute thermometry based upon the velocity of sound in helium gas , secondary thermometry involving principally studies of the behavior of germanium resistors , and helium 4 <WSD>vapor-pressure</WSD> measurements ( see p. 144 ) . 
Meaning of the word marked with <WSD> tags:  the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form)