Fault Masking is:

Fault masking occurs when one defect prevents the detection of another defect. In other words, an error condition hides another error condition, making it more difficult to identify and correct all defects. This can happen in various scenarios, such as when a certain path of code execution is blocked by an existing error, preventing other potential errors from being executed and detected. Reference: ISTQB CTFL Syllabus V4.0, Section 1.5.5​​.