The definition of mazel tov is the Hebrew expression of congratulations.(interjection)
An example of mazel tov is what is said after a Jewish wedding.
See mazel tov in Webster's New World College Dictionary
Origin: Heb (often via Yiddish) < māzal, luck + tōv, good
See mazel tov in American Heritage Dictionary 4
also ma·zal tovinterjection
Origin: Mishnaic Hebrew mazzāl ṭôb, good luck
Origin: : mazzāl, constellation, destiny (ultimately from Akkadian manzaltu, mazzaztum, position of a star, from izuzzu, to stand; see ḏwḏ1 in Semitic roots)
Origin: + ṭôb, good; see ṭyb in Semitic roots.