"Dead code" refers to sections of a computer program that exist but are never executed or used. This can happen when the code becomes unnecessary due to changes or restructuring of the program but is not removed. Even though it has no direct function, dead code can make the program unnecessarily complex, harder to maintain, and, in some cases, slightly affect performance.

Common causes of dead code include:

1. Outdated functions or methods: Functions that were once used but are no longer needed.
2. Unreachable code: A section of code that can never be reached due to a prior return statement or condition.
3. Unused variables: Variables that are declared but never utilized.

Developers often remove dead code to improve the efficiency and readability of a program.