Sentence:  Thus the total <WSD>helicity</WSD> "H" equals the sum of the entries in a matrix "H"<sub>ij</sub>. If "N" is large then there will be many more mutual <WSD>helicity</WSD> terms. In this case ignoring the self <WSD>helicities</WSD> (if they are difficult to observe) may only give a small error. 
Meaning of the word marked with <WSD> tags:  Any of certain measures of the extent to which vortex lines (in fluid flow) or field lines (in a magnetic or electromagnetic field) kink and twist and/or link and coil around one another.