Sentence:  The Linear Mixing Model (LMM) assumes that the spectral signature of one pixel of the hyperspectral image is a linear combination of the <WSD>endmember</WSD> spectra corresponding to the aggregation of materials in the scene due to reduced sensor spatial resolution. Given the <WSD>endmembers</WSD>, unmixing algorithms compute the abundance coefficients providing sub-pixel resolution segmentations of the image. 
Meaning of the word marked with <WSD> tags:  A member at one end of a range or series made up of similar members.