Final answer:
The abbreviation 'prn' stands for 'pro re nata', meaning 'as needed'. It is used in medical prescriptions to specify that medication should be taken only when necessary.
Step-by-step explanation:
The abbreviation prn stands for 'pro re nata', which is a Latin phrase that translates to 'as needed' or 'as the situation arises'. It is commonly used in medical prescriptions to indicate that the medication should be taken when the patient feels it is necessary, rather than at regularly scheduled times. For instance, if a patient is given pain medication with the instruction to take it prn, they would use the medication when they experience pain, up to the maximum dosage as specified by the healthcare provider.