To congratulate is to praise or compliment someone or to offer them good wishes when something good happens.(verb)
When you say "Great job, good for you!" to someone who is graduating, this is an example of a situation where you congratulate the graduate.
See congratulate in Webster's New World College Dictionary
Origin: < L congratulatus, pp. of congratulari < com,- together + gratulari, to wish joy < gratus, agreeable: see grace
See congratulate in American Heritage Dictionary 4
transitive verb con·grat·u·lat·ed, con·grat·u·lat·ing, con·grat·u·lates
Origin: Latin congrātulārī, congrātulāt-
Origin: : com-, com-
Origin: + grātulārī, to rejoice (from grātus, pleasing; see gwerə-2 in Indo-European roots).
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