F is defined as the sixth letter of the English alphabet, the Fahrenheit temperature scale, a grade for failing work, or the fourth note in the C musical scale. "Press F to pay respects" is the caption in a scene that occurs at a funeral in Call of Duty: Advanced Warfare. There are a variety of ways. plural F's or Fs, f's or fs. We now have eff and effing as well as f-word and f-bomb, all of which allow us to discuss the term without resorting to its actual use. F definition: F is the sixth letter of the English alphabet.