Saying “stop” as the result of seeing a red octagonal sign with no letters on it is another example of metonymical extension. The shape and color are irrelevant stimulus features that frequently accompany the written word “stop.”
An instance of verbal behavior with the following features:
The response form has already been acquired in one or more of the elementary verbal relationships
The stimulus must be novel
The novel stimulus must have none of the relevant features of the class of stimuli that previously controlled the response