1. The relative position of two things that are next to each other: The vases were placed in apposition to each other on the shelf.
2. The quality of being side by side, not being front-to-front, but next to each other: The members of the marching band stood in apposition to each other as they prepared to play the national anthem.
3. A placing of two things conjointly or the fitting together of two things: The two pieces of the puzzle fit in perfect apposition.
4. A grammar relationship between noun phrases in which the relationship between two usually consecutive nouns or noun phrases that refer to the same person or thing and have the same relationship to other sentence elements: In the sentence, "My son, an actor, lives with me"; the phrase, "My son, an actor" is an example of an apposition.

5. In physiology, cell growth in which layers of material are deposited on existing ones: Dr. McMahon told Henry that the growth on the bottom of his heel was an example of cellular apposition.