vanishing point

noun2

1. •
   The point in a perspective drawing at which parallel lines receding from an observer seem to converge.

   • These formulae . . . give for the vanishing point of a set of parallel lines whose common azimuth is α and declivity δ: η=d tan α, ζ=d tan δ sec α. — 1884
2. •
   The situation in which, place where, or point in time when some object or phenomenon is no longer observable or notable.

   • It might cut the tuberculosis deathrate to the vanishing point. — 1923