Sentence:  They took figures for the abundance of invasive weeds mapped according to the normal grid unit of the 'hectad', or 10 × 10 km square, and then looked at how abundant these species were mapped at a much finer scale – in &#39;<WSD>tetrads</WSD>&#39;, or 2 × 2 km squares, inside these hectads. 
Meaning of the word marked with <WSD> tags:  A unit of land area of two by two (that is, four) square kilometres.