"Vague or Undefined Terms" (or Glittering Generalities as I call it when I teach propaganda) is defined as challenging an opinion by using words that are so vague or so poorly defined as to be almost meaningless.
In Animal Farm, there could be several examples of this. One would be their motto,"All animals are Equal". Equality is a hard to define word. What could be equality for one society might be seen as favoritism by another.
Another example might be another motto on the farm of "Four legs good, two legs bad" The words good and bad are so vague that they really mean nothing. What is good? According to whom? What is bad?
And another example might be Minimus' song he wrote for Napoleon insinuating that Napoleon was the reason for everything positive on the farm.