The Collaborative International Dictionary of English v.0.48:

Rhaetic \Rhae"tic\, a. [L. Rhaeticus Rhetian.] (Geol.)
   Pertining to, or of the same horizon as, certain Mesozoic
   strata of the Rhetian Alps. These strata are regarded as
   closing the Triassic period. See the Chart of Geology.
   [1913 Webster]