Based on the name, you can assume that a workplace injury is something that happens when you are at work. That is partially correct.
The Occupational Safety and Health Administration explains workplace injuries have some specific requirements.
Work environment
You must suffer your injury or illness as a result of exposure or events that occur within your work environment. OSHA explains the work environment is the location in which you conduct your job duties. It also includes anywhere you may be when conducting work, even if it is not your usual workplace.
On duty
To be a workplace injury, it must happen while you are working during the course of your regular duties. You need to be on the clock, in other words. It is important to note that if you are not working or you are not injured as a result of doing your job, then it is not a work injury.
Illnesses
If you get sick while at work because of a contagious disease, it may be a valid injury. The common cold and flu are exempt from this. But other diseases of a more serious nature may be work-related injuries if you can show you caught the infection at work.
There are always gray areas, especially if the claim is for an illness or aggravation of a pre-existing condition. While these may be covered, it can be difficult to prove that your job duties or work environment led to them. You may need to show medical records to prove your claim. It is easiest to prove injuries resulting from an accident at work.