Definition of "flagstone"

Grass sprouted between the pebbles and a greenish stain lay upon the flagstones.

1891, Sir Arthur Quiller-Couch, The Blue Pavilions

In 1946, several of the heavy flagstones, which form the decking of the viaduct, were replaced or reseated, and manholes were provided in the decking, to facilitate access to the inner walls.

1949 May and June, “Notes and News: First Repairs in a Century”, in Railway Magazine, page 202