WordWeb Online

1. Producing or capable of producing an intended result or having a striking effect
   "his complaint proved to be effectual in bringing action"
   - effective, efficacious
    
2. Having legal efficacy or force
   - legal, sound

See also: effectiveness, effectivity, effectuality, effectualness, efficacious, efficient, hard-hitting, impelling, impressive, potent, powerful, rough-and-ready, stiff, strong, telling, trenchant, useful, utile, valid