WordWeb Online

1. Not being done or happening often or much; happening from time to time
   "took an occasional glass of wine"
    
2. Occurring or appearing at usually irregular intervals
   "occasional headaches";
   - episodic, episodical
    
3. Not regular; working on an informal or ad hoc basis
   "an occasional worker";
   - casual
    
4. Recurring or reappearing from time to time
   "occasional feelings of anxiety";
   - periodic

See also: infrequent, irregular, sporadic, unpredictable