thin

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Related to thining: thinning out

thin

1. (of a photographic negative) having low density, usually insufficient to produce a satisfactory positive
2. Mountaineering a climb or pitch on which the holds are few and small
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

thin

[thin]
(meteorology)
In aviation weather observations, the description of a sky cover that is predominantly transparent.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.