Final answer:
The decline in the ability to recall a previously learned sequence of letters after performing a distraction task is known as retroactive interference.
Step-by-step explanation:
The phenomenon described in the experiment where subjects learned a sequence of letters and then showed a rapid decline in their ability to remember the letters after counting backwards is known as retroactive interference. This type of interference involves the situation where information learned more recently hinders the recall of older information.
In the given scenario, the distraction of counting backwards by threes is a task that occurred after learning the letter sequence, thereby interfering with the subjects' ability to recall the previously learned letters. Retroactive interference happens in various situations, such as when learning new material in a class makes it difficult to remember material from a previous class.