Thin Section Meaning

(mineralogy) A thin, optically flat sliver of a material, especially a rock or mineral, that can be used in microscopy.

Example: 1980, W. S. Mackenzie and C. Guilford, Atlas of rock-forming minerals in thin section, p. iv:
  The purpose of this book is to illustrate the appearance of many of the common rock-forming minerals in thin section under the microscope.
2003, Davis Young, Mind over Magma: The Story of Igneous Petrology, p. 150:
  Before long, as Dawson (1992) pointed out, Sorby (1863) also made thin sections of some of the fused charges of basalt produced by Sir James Hall.
Used other than as an idiom: see thin,‎ section.