An application is subjected to a constant load for an extended period of time as...

In the ISTQB terminology:

A defect (also known as a bug or fault) is an issue in the code that can cause a failure.

A failure is the observable incorrect behavior or output of the software due to a defect.

In this scenario, the memory leak is the defect in the application code that causes the application not to release dynamically allocated memory properly. The observable effect of this defect, which is the slowdown in response time, is considered a failure because it is the manifestation of the defect that prevents the application from meeting its performance requirements.