An imaginary line running through an aircraft from nose to tail.

The axis extending through the center of the fuselage from the nose to the tail. The longitudinal axis is sometimes called the "x" axis. Roll is a motion around the longitudinal axis.

An imaginary reference line running down the center of the aircraft between the nose and tail.

The spatial dimension of an estuary, such as San Francisco Bay, that extends from the landward to the seaward domains; the USGS makes water quality measurements along the longitudinal axis of the Bay-Delta, from the Sacramento River, to the Central Bay, and into the South Bay

Axis from head through the body (like a skewer) around which twisting is done

An imaginary line dividing the rink into two equal halves parallel to the long dimension of the rink.

Axis along the length of the member passing through the centroid.