From the Trauma Nursing Core Course Provider Manual, 6th Edition:
Trauma is any injury to the body caused by transfer of energy from the outside environment, such that the energy transferred into the body exceeds the body's ability to absorb said energy. That energy can be kinetic energy such as that imparted by acceleration or deceleration (i.e. motor vehicle crashes, pedestrian vs. vehicle incidents, falls, etc), that imparted by a blunt object (beatings, crush injuries, falls onto objects, etc), that imparted by a sharp object (stabbings, impalements, gunshot wounds, etc); it can be thermal energy, such as that imparted by a fire (burns); electrical energy (electrocutions, lightning strikes); chemical energy (burns, accidental or intentional ingestions); or radiant energy (UV burns, ionizing radiation exposures). Drownings and smoke inhalations are instances of trauma in which the mechanism of injury is a lack
of energy - specifically oxidative injury resulting from anoxia.
From the emergency healthcare perspective, "trauma" refers to a severe injury or injuries to the body requiring immediate and intensive management. Frequently (though not always!) they present to the care setting via EMS, and their immediate care consists of ensuring airway patency, effective respiratory function, hemodynamic stability and neurological function until definitive care (usually surgical) can be provided.
Not every emergency facility has the staff or the resources to provide that definitive care. There is an accreditation process for trauma centers, overseen by the American College of Surgeons, which divides emergency facilities into Level I (most comprehensive, specialty staff on-site 24/7, also conducts trauma research) through Level III (least comprehensive, basic services available, has agreements with Level I/II centers for trauma transfers). When trauma cases present to low-ranked or unaccredited facilities, the staff there will provide immediate care in preparation for transfer to a higher-ranked facility.
If trauma is something that interests you, I'd recommend going to a medical library and finding a copy of the TNCC manual. It's everything you need to know about trauma nursing in one package.
Hope this helps!