Sandvich is the snack that Heavy eats. In many of Kitty0706's videos, He has mentioned the word "sandvich".

The Sandvich in it's entirety.

Sandvich is simply the way Heavy says "sandwich" due to his Russian accent, and the term is used for Heavy's sandwiches overall, not only on Kitty's videos, but almost the entire TF2 fanbase. It is official that it is called Sandvich instead of sandwich, based on the "Meet The Sandvich" video by Valve themselves.

Sandvich is used in the game TF2 for Heavy to eat to raise his health. When he eats it he says "Sandvich, om nom nom, nom nom".
Meet the Sandvich

Meet the Sandvich

The "Meet The Sandvich" original video.