1. The second cervical vertebra in higher vertebrates; the first vertebra of amphibians.

2. The center line of an organism, organ, or other body part.

1. A line where a folded bed has maximum curvature.

2. The central portion of a mountain chain.

The locus of intersection of two pencils of lines in perspective position.

1. In a coordinate system, the line determining one of the coordinates, obtained by setting all other coordinates to zero.

2. A line of symmetry for a geometric figure.

3. For a cone whose base has a center, a line passing through this center and the vertex of the cone.

A line about which a body rotates.