Hemagglutinin is a glycoprotein that is commonly found on the membrane of influenza viruses. This protein is involved in binding the virus to host cells and triggering the infection. There are several synonyms for hemagglutinin, including viral surface protein, spike protein, and HA protein. These terms all refer to the same protein, but are used in different contexts and by different researchers. Hemagglutinin is a critical component of the influenza virus, and its function is highly conserved across different strains and subtypes. Researchers continue to study this protein in order to develop new treatments and vaccines to combat influenza.