A piece of food consisting of two pieces of bread with a filling of some sort, originating, though not limited to Earth.
References
The Third Doctor eats one of the Master's sandwiches during a sword fight with him. (DW: The Sea Devils)
Whilst meeting with Colonel Trenchard the Third Doctor eats a plate full of sandwiches, originally ordered by Jo Grant. (DW: The Sea Devils)
Charley Pollard, Viola Learman, Major Ferdinand and the Eighth Doctor make and eat sandwiches whilst being held by the Daleks in Mariah Learman's palace in New Britain. (DW: The Time of the Daleks)