Question: Who is Turnip Head?
The term "Turnip Head" was first introduced in Episode 1x21, The Greater Good.
Answer: Claire was afraid that something was going to happen to her baby, so she didn't name him. It wasn't until Danielle Rousseau took him, that Claire named him Aaron. Previously to that, Charlie called him Turnip Head because his head looked like a turnip (Ep: 1x21, The Greater Good).