The word for today is crux, meaning "cross." This is a feminine noun of the third declension, with a genitive form crucis. You can see the Latin word borrowed directly into English with the word "crux" (as in "the crux of the matter") which derives from the Latin phrase crux interpretum, meaning "a crossways of the interpreters," a passage in a text where interpreters disagree. The Latin root is also clear in words like "crucial," "crusade," "excrutiating," etc. The English word "cruise" even derives from Latin, via Dutch kruisen, meaning "to cross over, sail back and forth." We get "cross" from Old Irish cros that ultimately derives from Latin crux.
The original meaning of the word was a tree or some other wooden device used for the purposes of execution, impaling or hanging condemned prisoners. Then, by metaphorical extension, it came to refer to any kind of trouble or suffering that someone could endure.
The etymology of the word in Latin is not clear. Although this is certainly a productive root in Latin, generating many related words (crucifigo, excrucio, crucifer, cruciabundus, to name just a few), nobody is quite sure just where the word crux came from to begin with. Since the word refers to what was a distinctively Roman practice, it does not have a Greek language counterpart. In the Greek New Testament, the word for cross is σταυρός (which meant a stake or a pale, as would be used for impalement), which is etymologically unrelated to Latin crux.
Here are some examples of today's word in Latin sayings and proverbs; for more information, see the page at the Scala Sapientiae, which contains notes on some of the proverbs cited below, as well as additional proverbs:
I in malam crucem!
Abi hinc in malam crucem!
Destinatus cruci non submergitur.
Destinatus cruci non suffocatur aquis.
Ille crucem pretium sceleris tulit, hic diadema.
This word took on entirely new dimensions of meanings from its central role in the Christian religion, because of the crucifixion of Jesus. As a paradoxical symbol of salvation and goodness, the Christian meaning of the crux is very different from the traditional Roman meaning.
Ubi crux, ibi lux.
Per crucem ad lucem.
Via crucis, via lucis.
Quisque suas sustinet cruces.
Crux in pectore, in corde Satan.
Malus fugit lucem, ut diabolus crucem.
Christi crux est mea lux.