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. C-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.