not really. a turndown is when you turn your bars and push them down, hence "turn-down". a lookback is when you turn your bars and pushe your deck out alot, more than in a turndown.
What Kieran said is right, lookbacks are always FS and turndowns are BS... Also ^^^ Lookbacks don't have to be in a spin, you can do lookbacks in FS airs and hips.