The change in morphology and behavior of an endothelial cell resulting from exposure to a cytokine, chemokine, cellular ligand, or soluble factor. [ http://www.ncbi.nlm.nih.gov/pubmed/14581484 https://www.worldcat.org/search?q=bn%3A0781735149 GOC:mgi_curators http://www.ncbi.nlm.nih.gov/pubmed/12851652 ]