We'll start with the basics. Travel insurance is a generic term that refers to medical insurance for travelers. These travelers can be U.S. citizens or foreign. Travel medical insurance and travel health insurance are simply synonyms of travel insurance.
Visitors insurance is a type of medical insurance that is specifically designed for foreign visitors to the United States. Visitors health insurance and visitors medical insurance are just synonyms of visitors insurance.
Trip insurance, on the other hand, will not typically offer coverage for medical expenses. Instead, trip insurance covers expenses related to the trip itself, such as trip cancellation, interruption or postponement, lost baggage, stolen passports, etc.
One thing to keep in mind is that while some travel insurance plans do include both medical coverage and trip protections, your basic trip insurance will not cover any medical expenses if an injury or illness is incurred while abroad.