any of various cells that destroy germs, infected cells, etc.; esp., a lymphocyte () that destroys infected or cancerous cells by releasing proteins that damage the cell membranes of the target cells
See killer cell in American Heritage Dictionary 4
Any of various large differentiated T cells that recognize and lyse target cells bearing a specific foreign antigen, usually by infection with a virus, and that also function in defense against cancer cells. Also called cytotoxic T cell, killer T cell.