True patient abandonment occurs when you have accepted the care of your patient assignment and then for some reason leave without first making sure that the responsibility for their care has been appropriately transferred to another qualified provider.
Some scenarios where I have read or seen this to occur is when the nurse feels the care is inappropriately weighted as too heavy and goes home after informing the charge nurse before they can make accommodations in shifting the assignment to make sure patients are cared for. Another scenario is leaving before the supervisor can get appropriate relief in to cover your patients (such as during inclement weather, or when a family emergency arises).
True patient abandonment can have severe consequences to both the nurse and the patient which is why you must always be sure your assignment gets a proper handoff before you leave.
Many state Practice Acts address this issue in their regs too.