Milestone

A milestone was a stone which marked the path to some destination, like a village, indicating proximity. Often bearing an inscription, these stones were placed alongside roads, to guide travellers along their way.

As evidence of history, standing still even as the landscape around them changed, the Third Doctor found some of these markers "fascinating". (PROSE: Doctor Who and the Sea-Devils)

Examples
On the island where was being held prisoner, the Third Doctor pointed out an old milestone, which pointed out a way, in the fork in the road, to a settlement which had long fallen under the sea.

Clearing up the moss, which had collected over time, with a wire brush, produced from his pockets, he uncovered a name: Belial Village. Tired from walking, Jo Grant did not share the Doctor's enthusiasm about this ancient marker. (PROSE: Doctor Who and the Sea-Devils)

In Colin's copy of the Ladybird storybook Dick Whittington, Dick Whittington and his cat travelled along a dirt track to a city, passing many milestones. (PROSE: To the Devil — a Diva!)

Figurative sense
In the real world, "milestone" is often used to mark significant events, which delineate different eras or highlight personal achievements, for example. Indeed, by some accounts, this page was a milestone for marking 100,000 pages on Tardis Wiki. (Mileage may vary.)

Doctor Who milestones

 * In the Editors' Notes for Decalog, the anthology was referred to as "a milestone in Doctor Who's literary history [...] [as it was] the first bona fide short-story anthology ever to be published."
 * DWM 500 had several examples of Doctor Who milestones.
 * Fan David McKinlay called five hundred issues of DWM a milestone in Pick of the Penguin.
 * DWM 582 had several examples of Doctor Who milestones.
 * Jonathan Morris wrote in LOG! that DWM 50 celebrated its milestone by printing a list of every Doctor Who story, that DWM 300 celebrated its milestone by including a free CD, and that DWM 400 was a milestone.
 * In 500 DWM Issues, Cefn Ridout referred to DWM 100 as a milestone. It was also noted that 2015 was a milestone as it was the tenth anniversary of the 2005 era of Doctor Who.
 * As revealed in "The big 500! There another or better final interview...", Tom Baker also referred to the five hundredth issue of DWM as a milestone.
 * In the Welcome section of DWM 582, editor Marcus Hearn referred to The Power of the Doctor as a "milestone episode".
 * In the Ruff Mix article of DWM 582, it was said that the Doctor Who theme was "a milestone in electronic music."