A testable requirement is a requirement that has been broken down to a level where it is precise, unambiguous, and not divisible into lower level requirements.  These criteria are only met if it is possible to write a test case that would validate whether the requirement has or has not been implemented correctly.  This is the source of the term "testable requirement".  For most requirements, a testable requirement could be described in terms of:

The state of the system and the data elements that are inputs (e.g., customer number, product number)

The condition or action associated with the requirement (e.g., the user enters data, the order is validated, the check amount is deducted)

The expected or specified result described in terms of data elements (e.g., customer number must be 8 digit numeric, product quantity must be greater than zero).

Visio 2000 Drawing

Figure 3