impulsive

Short definition from Wiktionary

Adjective

1. 1
   Having the power of driving or impelling; giving an impulse; moving; impellent.
2. 2
   Actuated by impulse or by transient feelings; inclined to make rapid decisions without due consideration.
3. 3
   Acting momentarily, or by impulse; not continuous – said of forces.

Noun

1. 1
   That which impels or gives an impulse; an impelling agent.
2. 2
   One whose behaviour or personality is characterized by being impulsive.