Course of employment is a legal consideration of all circumstances which may occur in the performance of a person's job, especially during a period of time where specific objectives are given by the employer to the employee. Black's Law Dictionary (2nd Pocket ed. 2001) pg. 154.
Key examples of this consideration under US law can include tort liability or ownership of intellectual property. If an employee is driving a motor vehicle during working hours and harms the person or property of another, a court would consider course of employment to determine if the employer had vicarious liability for the harm. Extreme examples would likely find the employer is liable for a truck driver on her assigned route but not for a secretary picking up his child from day care.
Similarly the employer would likely own the copyright rights to a song written by an employee who was hired as a composer, but not if the employee was hired as an accountant.