An important event in the development or history of something or in someone's life.

A state in a child's development at which they can do a particular thing for the first time.

A state in a project that can be used to measure progress towards its final goal.

A stone at the side of the road that shows the distance to various places, especially to the nearest large town.

A time when a piece of work must be finished or something must be achieved.

Good service is one touchstone of a first-class restaurant.

Since the early 16th century, touchstone has referred to a particular kind of siliceous stone (that is, stone containing silica) used to do a particular job: determine the purity of precious metals. The process involves comparing marks made by rubbing a sample of a metal of known purity to marks made by a metal of unknown purity. The method is accurate enough in the case of determining the purity of gold that it is still in use today. Figurative use extended from this literal use, with touchstone functioning as a word for a test or criterion to determine the quality of a thing, and later to refer to a fundamental or quintessential part or feature of something.

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a time when a piece of work must be finished or something must be achieved: