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What is the difference between congratulation and congratulations?

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From NOAD:

congratulation |kənˌgra ch əˈlā sh ən; -ˌgrajə-| noun an expression of praise for an achievement or good wishes on a special occasion; the act of congratulating : he began pumping the hand of his son in congratulation. • ( congratulations) words expressing congratulation : our congratulations to the winners | [as exclam. ] congratulations on a job well done!

You make the call.

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It is difficult to articulate the difference between them.

When you are speaking about the act of praise, a single act of praise is a congratulation, multiple is -s. You can almost always use the plural form and sound perfectly normal.

When you wish to praise someone:

I congratulate you. (much more formal, can easily sound distant or impersonal)

You have my congratulations. Shortened: Congratulations! or Congratulations on your wedding! (always plural in shortened forms seemingly expressing multiple praises, explanation for what the congratulation is for only necessary when it isn't clear)

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The difference is very subtle :

  • "congratulation" is congratulating, that is having some joy, some pleasure, perhaps by yourself and even secretly, for instance because you have won at the lottery ;

  • congratulations" are the expression of that, for instance to a newly married couple.

The word comes from Latin congratulatio, with exactly the same meaning.

  • OED says your first sense, "Grateful and glad acknowledgement on one's own behalf, rejoicing" is obsolete. I suppose self-congratulation might not be, but that's related to your second sense. – Andrew Leach Oct 14 '13 at 9:34