momentum

Momentum, momenti is second declension neuter, contracted out of movimentum from moveo. That origin explains the odd pair of meanings: it is the smallest push that makes a balance tip, and from there both "the instant in which the scale moves" and "the weight that decides an issue".

The importance sense almost always shows up as a genitive of value: magni momenti, "of great importance", nullius momenti, "of no account". The time sense sits in the ablative, momento, "in a moment". English has taken both halves and split them into different words, moment and momentous, and then physics quietly borrowed the Latin word back in its original mechanical meaning, which is tidier than English usually manages.