The crossword solution for the term 'Word Introducing an Alternative' is UNLESS. This word is used to introduce a condition or situation that would cause something else to happen. For example, 'I will go to the store UNLESS it's raining.' In this sentence, the speaker is saying that they will go to the store UNLESS there is rain, in which case they will not go. UNLESS is a useful word for introducing alternatives and conditions in conversations and writing.