What is the meaning of the word "serration"?

Definitions:

  1. a row of notches copy to clipboard noun
    • the pliers had serrations to improve the grip copy to clipboard
  2. a single notch in a row of notches copy to clipboard noun
    • one of the serrations was broken off copy to clipboard
  3. the condition of being serrated copy to clipboard noun
    • the serrations of a city skyline copy to clipboard