An unreferenced variable in the source code of a computer program is a variable that is defined but which is never used.
This may result in a harmless waste of memory.
Many compilers detect such variables and do not allocate storage for them (i.e., "optimize away" their storage), generally also issuing a warning as they do.
[1] Some coding guideline documents consider an unreferenced variable to be a symptom of a potential coding fault.
On the other hand, unreferenced variables can be used as temporary placeholders to indicate further expected future developments in the code.