A small concavity is a NICHE, which is a shallow recess or indentation in a wall or other surface. It is typically used to provide a decorative feature or to hold an object such as a statue or vase. Niches are often found in churches, temples, and other religious buildings, as well as in homes and public spaces. They can also be used to create a focal point in a room or to provide a place for a candle or other light source.