It is a description of how the diamond was cut. The basic rose cut has a flat base (no pavilion) and a crown composed of triangular facets (usually 12 or 24) in symmetrical arrangement, which rise to form a point.

As @Lightlyseared said, it is how a diamond is cut. Here is the history of such a diamond

Older cut diamonds were often like this before the 58 facet “brilliant” became a standard for a round cut stone.