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

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

Encyclopedia: Impulsive