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dom·i·no 1 (dŏmə-nō)
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n. pl. dom·i·noes or dom·i·nos
1.
a. A small rectangular wood or plastic block, the face of which is divided into halves, each half being blank or marked by dots resembling those on dice.
b. dominoes or dominos (used with a sing. or pl. verb) A game played with a set of these small blocks, generally 28 in number.
2. A country expected to react politically to events as predicted by the domino theory: "The dominos did indeed fall in Indochina" (Arthur M. Schlesinger, Jr.)

[French, probably from domino, mask, perhaps because of the resemblance between the eyeholes and the spots on some of the tiles; see DOMINO2.]
(click for a larger image)
domino1

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
 
dom·i·no 2 (dŏmə-nō)
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n. pl. dom·i·noes or dom·i·nos
1.
a. A costume consisting of a hooded robe worn with an eye mask at a masquerade.
b. The mask so worn.
2. One wearing this costume.

[French, probably from Latin (benedīcāmus) dominō, (let us praise) the Lord, dative of dominus, lord; see dem- in the Appendix of Indo-European roots.]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
 

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.