Adjacent means close to or near something. You may consider the people up and down your street to be neighbors, but your next door neighbor is the person who lives in the house or apartment adjacent to yours.
Adjacent can refer to two things that touch each other or have the same wall or border. And the adjective is often followed by the preposition to: Her office is adjacent to mine. This word is from Latin adjacere "to lie near," from the prefix ad- "to" plus jacere "to lie, throw."