http://www.cse.unsw.edu.au/~billw/cs9414/notes/prolog/intro.html
If Prolog finds a variable name that you only use once in a rule, it assumes that it may be a spelling mistake, and issues a Warning about a "singleton variable" when you load the code: 
% prolog -q -s mycode.pl
Warning: .../mycode.pl:4:
   Singleton variables: [Item]

http://www.cs.toronto.edu/~dianaz/2501/Manual/sec-B.html
singleton [variable]
Variable appearing only one time in a clause. SWI-Prolog normally warns for this to avoid you making spelling mistakes. If a variable appears on purpose only once in a clause, write it as _ (see anonymous) or make sure the first character is a _. See also the style_check/1 option singletons