a rock of fissile or laminated structure formed by the consolidation of clay or argillaceous material.

1740–50; origin uncertain; compare obsolete shale to split (said of stone), to shell, derivative of shale shell, husk, Old English scealu shell, husk; see scale2

shalelike, shaley, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
shale (ʃeɪl)
a dark fine-grained laminated sedimentary rock formed by compression of successive layers of clay-rich sediment
[Old English scealushell; compare German Schalstein laminated limestone; see scale1, scale²]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1747, possibly a specialized use of M.E. schale "shell, husk, pod" (c.1380), also "fish scale," from O.E. scealu (see shell) in its base sense of "thing that divides or separate," in ref. to the way the rock breaks apart in layers. Geological use also possibly influenced by
Ger. Schalstein "laminated limestone," and Schalgebirge "layer of stone in stratified rock."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
shale   (shāl)  Pronunciation Key 
A fine-grained sedimentary rock consisting of compacted and hardened clay, silt, or mud. Shale forms in many distinct layers and splits easily into thin sheets or slabs. It varies in color from black or gray to brown or red.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

shale definition

A sedimentary rock formed from layers of clay.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Fracking is a form of drilling for natural gas from shale rock.
The excavation soon struck slabs of shale that had likely been thrown in to
  fill up what was clearly a burial site.
Halfway up, it began to drizzle, and he grew anxious about her trainers on the
  wet shale near the top.
Our experience with shale gas shows us that the payoffs on these public
  investments don't always come right away.
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