If something is odious, it's hateful. If you become a historian of slavery, you'll learn all the details of that odious trade.
Odious is from the Latin noun odium, which means hatred. It is a strong word, so don't call someone odious unless you want to accuse someone of being loathsome or vile. Actions can also be called odious. A typical use is Shakespeare's in Othello: "You told a lie, an odious damned lie." Some synonyms are hateful, contemptible, detestable, and abominable.