input: Write a python function to get the definition of a word using the Merriam-Webster API.
  ```python
  assert get_definition("Ambitious") == "having a desire to achieve a particular goal"
  assert get_definition("Python") == "a large heavy-bodied nonvenomous constrictor snake occurring throughout the Old World tropics."
  assert get_definition("NonexistentWord") == "No definition"
output: import requests
  
  def get_definition(word):
      api_key = 'your_api_key_here'
      url = f"https://www.dictionaryapi.com/api/v3/references/collegiate/json/{word}?key={api_key}"
      response = requests.get(url)
      data = response.json()
      
      if not data or isinstance(data[0], str):
          return "No definition"
      
      try:
          definition = data[0]['shortdef'][0]
      except (IndexError, KeyError):
          return "No definition"
      
      return definition