The crossword solution for the term 'CERTAIN PREPOSITION' is 'INTO', which is a four-letter preposition. Prepositions are words that link nouns, pronouns, and phrases to other words in a sentence. They usually indicate location, direction, or time. The preposition 'INTO' is used to indicate movement or direction towards the inside of something. It can also be used to indicate a change in state or condition. For example, 'She walked INTO the room' or 'He changed INTO his pajamas'.