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/12851652 http://www.ncbi.nlm.nih.gov/pubmed/14581484 GOC:mgi_curators http://openisbn.com/search.php?q=0781735149&isbn=1/ ]