The term 'option word' is used to refer to two words that can be used interchangeably in a sentence. The two words are 'EITHER' (6 letters) and 'ELSE' (4 letters). Both words can be used to indicate a choice between two or more possibilities, but they have slightly different connotations. 'EITHER' implies that the speaker is offering a choice between two options, while 'ELSE' implies that the speaker is suggesting an alternative option.