One celled critter. Flagellate protozoa are single-celled organisms which have whip-like threads extending from these cells which act to propel the organism thru its world. These are the flagella. Most common to medicine is trichomonas vaginalis, the fellow responsible for one type of vaginitis.
Context. I usually see that on wet prep exams. In that situation it is trichomonas. That is a sexually tranmitted protozoall infection. It causes a foul smelling vaginal discharge, and it is treatable with metronidazole.