Several algorithms use a predicate that must impose a strict weak ordering on pairs of elements from a sequence. For the predicate pr(X, Y):

"strict" means that pr(X, X) is false 
"weak" means that X and Y have an equivalent ordering if !pr(X, Y) && !pr(Y, X) (X == Y need not be defined) 
"ordering" means that pr(X, Y) && pr(Y, Z) implies pr(X, Z) 

From MSDN