I have heard the ten commandments written in Exodus and Deuteronomy sometimes called the Decalogue. What does this word mean and why are they sometimes called that?
The English phrase "Ten Commandments" is a bit of a misnomer. The Hebrew phrase עֲשֶׂרֶת הַדְּבָרִים (eseret ha-dvarim) literally translates into English as "the ten words" or "the ten statements." The Hebrew phrase is translated into the LXX as δέκα λόγους (deka logous). Of course, from thence is derived the English expression "dekalogue" --- which is nothing more than a loose transliteration of that Greek phrase. δέκα λόγους is actually in the accusative case. The nominative would be δέκα λόγοι (deka logoi).
Decalogue comes from 'dekalogos', where 'deka' means 'ten', and 'logos', which is 'word', or in this context, 'statements'.