Dictionary.com
definitions

opposable thumb

[ə-pō′zə-bəl]

A thumb that can be placed opposite the fingers of the same hand. Opposable thumbs allow the digits to grasp and handle objects and are characteristic of primates.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
opposable thumb in Culture

opposable thumb

A thumb that can be used for grasping.

Note

Opposable thumbs are one of the distinguishing features of primates.

Note

The opposable thumb of human beings allows us to use tools.
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.