Defined:

Any argument where IF the premises were true, then the conclusion must be true.

We say an argument is "valid" if and only if it is a deductive form. The truth (or falsity) of the premises is irrelevant.

It is possible for an argument to be valid and untrue at the same time. Let's look at the following argument to illustrate: "If it is raining, then it is cloudy. It is raining. Therefore, it is cloudy." Look outside. It may not be raining right now. But the argument is still a valid deductive argument, because IF the premises WERE true, then the conclusion would have to be true. In other words, there is no possible way for the premises to be true and the conclusion to be false. If it's not raining right now, pretend. Can you imagine it raining without at least a small cloud? No, that's not possible. So we know that the argument is valid.

Invalid arguments are all those which are not valid. That is to say, all inductive arguments are (deductively) invalid.