A dog is dangerous if they have bitten a person or another dog with enough force that they have broken the skin. A dog that has broken the skin does not have reliable bite inhibition and is likely to do so again.
If you have a dangerous dog your first priority should be safety and managing your dog's environment and proximity to others in such a way that you can be sure that no one will get hurt.
Your next priority should be to immediately seek help from an Animal Behavior Consultant or Professional Dog Trainer. For safety reasons, we would not feel comfortable offering advice without first properly evaluating your dog in person.
To find a Certified Dog Trainer in your area, please consult the trainer-search databases of The International Association of Animal Behavior Consultants, or The Certification Council for Professional Dog Trainers. Both of these organizations have standards and accreditation requirements for all of their members.