Sentence:  For protein identification , fractions from the column were concentrated by pervaporation against a stream of air at 5 ` C or by negative <WSD>pressure</WSD> dialysis in an apparatus which permitted simultaneous concentration of the protein and dialysis against isotonic saline . 
Meaning of the word marked with <WSD> tags:  the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit)