Sentence:  If "A" is a skew symmetric matrix then det "A" is the square of a polynomial in the entries of "A". This polynomial is known as the <WSD>"Pfaffian</WSD>" of "A", and we denote it by Sym(pf "A"). 
Meaning of the word marked with <WSD> tags:  The determinant of a skew-symmetric matrix, capable of being written as the square of a polynomial in the matrix entries.