input: Write a python function to get the definition of a word from a dictionary and return "Not found" if the word does not exist.
  assert get_definition('algorithm') == 'a set of steps used to solve a specific problem'
  assert get_definition('loop') == 'a construct used to iterate over a set of items'
  assert get_definition('python') == 'Not found'
output: def get_definition(word):
      dictionary = {
          'algorithm': 'a set of steps used to solve a specific problem',
          'loop': 'a construct used to iterate over a set of items',
      }
      return dictionary.get(word, 'Not found')