The definition of a sixth sense is an extra sense, often thought of as perception or intuition.
An example of a sixth sense is the ability to communicate with those who have passed on.
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a power of perception thought of as a sense in addition to the commonly accepted five senses; intuitive power
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
A power of perception seemingly independent of the five senses; keen intuition.
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. sixth sense - Phrases/Idioms
Keen intuition, as in
She had a sixth sense that they would find it in the cellar. This term alludes to a sense in addition to the five physical senses of sight, hearing, smell, taste, and touch. [c. 1800] The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.