Formal similarity is a relationship between a stimulus that evokes a response and the response-product of that response in which the stimulus and the response-product are both in the same modality (e.g., they are both visual or auditory) and their physical patterns or sequences resemble one another (e.g., they look or sound alike).
A relationship between a stimulus that evokes a response and the response-product of that response with the following features:
The stimulus and the response-product are both in the same modality (e.g., they are both visual or they are both auditory)
Their physical patterns or sequences resemble one another (e.g., they may look or sound alike)