The term 'LINES OF A PLANE' can be solved by the four-letter word 'AXES'. An axis is a line that passes through a fixed point and divides a plane into two equal halves. In geometry, an axis is a straight line that serves as a reference line for measuring distances in a plane. In three-dimensional space, an axis is a line that passes through a fixed point and divides a plane into two equal halves. In aviation, the lines of a plane refer to the three AXES of the aircraft: the longitudinal axis, the lateral axis, and the vertical axis. These AXES are used to control the aircraft's attitude, altitude, and speed.