noun (n)

• a force that moves something along(noun.phenomenon)

  Synonym:
  drift, impetus
  

  source: wordnet30
• the act of applying force suddenly(noun.act)

  Synonym:
  impetus, impulse
  

  source: wordnet30
• The act of impelling or driving onward, or the state of being impelled; the sudden or momentary agency of a body in motion on another body; also, the impelling force, or impulse.(noun)

  source: webster1913