webdriver.Firefox : WebDriverException: Message: 'geckodriver' executable needs to be in PATH

Question

>>> from selenium import webdriver
>>> browser = webdriver.Firefox()
**error Traceback** :

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/selenium/webdriver/common/service.py", line 76, in start
    stdin=PIPE)
  File "/usr/lib/python3.7/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.7/subprocess.py", line 1522, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'geckodriver': 'geckodriver'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/dist-packages/selenium/webdriver/firefox/webdriver.py", line 164, in __init__
    self.service.start()
  File "/usr/local/lib/python3.7/dist-packages/selenium/webdriver/common/service.py", line 83, in start
    os.path.basename(self.path), self.start_error_message)
selenium.common.exceptions.WebDriverException: Message: 'geckodriver' executable needs to be in PATH.