English

Noun

occult line ‎(plural occult lines)

1. A line drawn as a part of the construction of a geometrical figure or problem, but not to appear in the finished plan.
   • 1854, William Henry Smyth, The Mediterranean: A Memoir Physical, Historical, and Nautical, pp. 136-37 (Google preview):

     Yet in making an occult line on my chart, to indicate this rise from a depth at present beyond estimate, I by no means meant it as a mark for shoal water in which navigation is concerned.

References

• occult line at OneLook Dictionary Search