<programming> A variable
which is only referred to once in a piece of code, probably because of a programming mistake.
To be useful, a variable must be set and read from, in that order.
If it is only referred to once then it cannot be both set and read.
There are various exceptions.
-like assignment operators, e.g. "x += y", read and set x and return its new value (they are abbreviations for "x = x+y", etc).
A function argument
may be passed only for the sake of uniformity or to support future enhancements.
A good compiler
or a syntax
checker like lint
should report singleton variables but also allow specific instances to be marked as deliberate by the programmer.