Congratulation is what you offer when someone accomplishes something positive or has a happy life event. Applause after a performance, shouting "Way to go!" or sending a nice card — all express congratulations.
The noun congratulation comes from the Latin word congratulari, which means “with joy.” The word is usually used in the plural form: congratulations. Any achievement can bring you others' congratulations, from big things like receiving an award or graduating, to more everyday things like making the swim team or getting a good grade on a test. If someone is proud of you and happy for what you've done, congratulations are in order.