Idempotency is a property of mathematical functions: f : X -> X is idempotent iff f(f(x)) = f(x) for all x in X. Absolute value is an example of an idempotent function.
Reentrancy is a property of computational functions: a function f is reentrant if two copies of f can run at the same time (in the same memory space, etc. -- the details vary) and both copies behave as if the other weren't running. This can exclude or restrict, e.g. static fields or global variables.
In my mind, reentrancy is associated with threading, and implies that the code could be executing in multiple locations simultaneously, whereas an idempotent operation is one that can be repeated multiple times without negative side effects.
Reentrancy comes up with recursion and signal handling as well, both of which can come up in nominally single threaded programs (nominal because signals usually but don't always come from out of process).
