Final answer:
A lucid interval is when a client appears alert and oriented after a head trauma but then becomes somnolent, indicating the need for immediate medical attention.
Step-by-step explanation:
When discharging a client from the emergency department after a head trauma, the nurse teaches the guardian to observe for a lucid interval, which is most accurately described as an interval when the client is oriented but then becomes somnolent. This period is characterized by a temporary improvement in the client's condition where they appear alert and aware of their surroundings following the initial trauma, but then exhibit a decline in consciousness, possibly becoming drowsy or somnolent. It is a crucial observation because it may indicate a developing intracranial hematoma, a serious condition that requires immediate medical attention.