from Russian Google translate gave me: "cabbage stalk" BTW, in Russian it`s called very funny: "Kocheryzhka" -)

The stem.

It is called the core. Most people only eat the green leafy parts though. :)

I am not sure there is a special word for this part of the lettuce. If there is, I have never heard it. I think most often people will say "the leafy part of the lettuce" for the soft green leaves, and "the white part of the lettuce" or "crunchy part of the lettuce" for the inside.