In general terms, a centreline is a real or imaginary line through the centre of something, especially one that divides an object into two equal halves, thereby creating a mirror-like reflection of either side of the object.
- In a wave diagram used to illustrate electromagnetic waves, a centreline may be used to show either:
- The point where the electric and magnetic fields intersect and so illustrating the axis along which a wave propagates.
- A point halfway between the crest and trough of a wave and so illustrating the axis along which a wave propagates.