OK, I'll be another "Anybody else": as far as I understand your question bfisk has already given you the correct answer .
"Driftdown" is a specific term and is used in connection with an engine failure on a multi engine type, as in "what is the maximum driftdown altititude for a single engine failure at this weight".
"Letdown" is a less specific term, sometimes applied to instrument approaches, as in "given the weather, what's a good letdown to 27 left".