Addition word is a term used to describe words that are used to add two or more numbers together. The three words that are commonly used in addition are 'PLUS' (4 letters), 'AND' (3 letters) AND 'CARRY' (5 letters). PLUS is used to denote the addition of two or more numbers, while 'AND' is used to separate the numbers being added. Finally, 'CARRY' is used to denote the transfer of a number from one column to the next when adding multiple numbers.