IMMEDIATELY− Used by ATC or pilots when such action compliance is required to avoid an imminent situation. - FAA Controller pilot glossary1
The meaning of the word "immediately" is fairly imprecise. It does not specifically lay out within what parameters the pilot is expected to perform an instruction. It does imply a level of urgency, but the controller's interpretation may differ from the pilot's.
The relevant ICAO documents, ICAO Doc 4444 (Chapter 12) and ICAO Doc 9432 use the word “immediately” to imply urgency. ICAO recommends using the word "immediately" with the phrase "due traffic" when instant action is needed from the pilot.2
The term may also differ between different countries.
The UK Manual of Radiotelephony CAP 413. draws a distinction between “now” and “immediately” as follows:
“Use of the word 'now' indicates that the instruction should be complied with in accordance with normal aircraft operating procedures, but without delay. Use of the word 'immediately' indicates a further degree of urgency exists (e.g. to avoid flight into terrain or restricted airspace, or for the provision of collision avoidance, see Chapter 5 Paragraph 1.6.4 Avoiding Action Phraseology). In such circumstances, the pilot should take action to comply with the instruction as soon as practicable, subject to the safety of the aircraft.”3
Another word that can be used, which indicates a higher level of urgency is "expedite." It is usually used in terms of climb and descent, but after giving a turn instruction the controller can use the phrase "expedite turn" to indicate more urgency.
EXPEDITE− Used by ATC when prompt compliance is required to avoid the development of an imminent situation. Expedite climb/descent normally indicates to a pilot that the approximate best rate of climb/descent should be used without requiring an exceptional change in aircraft handling characteristics.1
A tragic example of the ambiguity of the word "immediate" is the mid-air collision between an F-16 and a Cessna 150 in 2015. When the controller noticed the conflict she gave the F-16 a traffic advisory and when the pilot did not appear to have a visual she gave the order "if you don't have that traffic in sight turn left heading 180 immediately."
Unfortunately the controller's expectation of the word "immediately" did not match the pilot's interpretation.
In postaccident interviews, the controller stated that when she issued the command to the F-16 pilot to turn left "immediately," she expected that the F-16 pilot would perform a high performance maneuver and that she believed that fighter airplanes could "turn on a dime."4
Instead the pilot began a standard-rate turn using the autopilot. This rate ended up being just enough to bring the F-16 directly into the flight path off the Cessna causing a mid-air collision that killed the two people aboard the Cessna. The NTSB report determined that "further clarification of her expectation, such as directing the pilot to "expedite the turn," would have removed any ambiguity."4
1 FAA Pilot Controller Glossary
2 Skybrary Urgency Instructions and Clearances
3 UK Manual of Radiotelephony CAP 413.
4 NTSB probable cause report:'14 CFR Armed Forces
Accident occurred Tuesday, July 07, 2015 in Moncks Corner, SC