Center.
Synonyms:
middle, midst.
We speak of the center of a circle, the middle of a room, the middle of the street, the midst of a forest. The center is equally distant from every point of the circumference of a circle, or from the opposite boundaries on each axis of a parallelogram, etc.; the middle is more general and less definite. The center is a point; the middle may be a line or a space. We say at the center; in the middle. Midst commonly implies a group or multitude of surrounding objects. Compare synonyms for AMID.
Antonyms:
bound, boundary, circumference, perimeter, rim.