WordWeb Online

1. Proceeding from natural feeling or impulse without external stimulus
   "an impulsive gesture of affection";
   - unprompted
    
2. Without forethought
   "letting him borrow her car was an impulsive act that she immediately regretted"
    
3. Having motive force; controlling or impelling
   "an impulsive force";
   - driving
    
4. Determined by chance or impulse or whim rather than by necessity or reason
   - capricious, whimsical, vagarious
    
5. Characterized by undue haste and lack of thought or deliberation
   "liable to such impulsive acts as hugging strangers";
   - hotheaded, impetuous, madcap, tearaway, brainish [archaic]

See also: arbitrary, dynamic, dynamical, incautious, self-generated, spontaneous, unpremeditated

Encyclopedia: Impulsive