1. in DSM–IV–TR, any of the dimensions that are helpful for describing individual behavior and thus facilitate clinical assessment. DSM–IV–TR (but not DSM–5) uses a multiaxial classification based on five axes: clinical disorders (Axis I), personality disorders and mental retardation (Axis II), general medical conditions (Axis III), psychosocial and environmental problems (Axis IV), and global assessment of functioning (Axis V).

2. an imaginary line that bisects the body or an organ in a particular plane. For example, the long or (cephalocaudal) axis runs in the median plane, dividing the body into right and left halves.

3. the second cervical vertebra, on which the skull rotates.

4. see neural axis.

5. a system made up of interrelated parts, as in the hypothalamic–pituitary–adrenal axis.

6. a fixed reference line in a coordinate system. See also abscissa; ordinate.