guidestone

noun2

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
2. •
   (figurative) Something 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