guidestone

plural guidestones

1. A traditional stone marker providing directions for travellers.

   Arrived at Wellington's Monument, the way levels over Eaglestone Flat and passes a guidestone […] that is unusual in being inscribed on one face only: Chesterfeild Roade.

   1980, Arthur Edward Dodd, Evelyn Mary Dodd, Peakland Roads and Trackways, page 108

2. figurativelySomething serving as a guide.

   Now, as a guidestone for modernization of the classical theory of meromorphic functions, a new revolution was born with discovery of two main theorems by R. Nevanlinna in 1925.

   1994, Mathematica Japonicae, volume 40, page 18