The sandwich we know today was created in 1762 and is credited to John Montagu, the 4th Earl of Sandwich.

The Earl was a problematic gambler and spent hours at the card table.   During one of his long days of playing, he worked up an appetite and requested something from the kitchen that he could eat with his hands without needing to leave his seat.  He was brought two pieces of bread filled with salt beef.

From that day, the sandwich evolved and became extremely popular.   

What defines a 'sandwich'?   According to Merriam Webster Dictionary, it is either two or more slices of bread or split roll, with a filling in between, or one slice of bread covered with food.    Interestingly, in some places, whether an item is a 'sandwich' is legally crucial, because such items may be taxed in different ways!