Abundant and copious are very, very similar adjectives. Abundant is the more commonly used word. Copious is a bit more formal. Abundant usually has positive connotations. It is rarely considered negative to have an abundance of something. Sometimes copious can have negative connotations, of being too much or too detailed. If you say someone took copious notes, it may suggest that they may have been more detailed than necessary and wasted some time. Cornucopia is a noun, unlike the other words that are adjectives. Usually, you will see it used like ¨there was a cornucopia of.....¨ meaning there was an abundance of, so it corresponds closer with abundance than abundant. Of those three words, abundant is the one you will likely want to use much more often than the others.