What is the definition of the morality in divine command theory? What does that mean to say that something is moral or something is immoral?
There's more than one theory that goes by the name divine command theory. I haven't seen the distinction drawn quite this way in the literature but there's an ontological divine command theory (hereafter DCT) that we can associate with the medieval idea of voluntarism (e.g., Ockham and following) and there's an epistemological DCT that has been offered recently by Robert Merrihew Adams and C. Stephen Evans. It might be best to think of these things as kind of a sequence.
On ontological DCT, something is right or wrong because and only because God has declared it so. Most readers take this position to be absurd or at least somewhat troubling, because it leads to or at least seems to lead to worlds where God says killing people is not only not wrong but is mandatory. Thus, to know morality is to know what God is saying which is arbitrary and do what he says.
Epistemological DCT shifts the purpose of divine commands relative to human subjects. For epistemological versions of DCT, what happens is that we know right and wrong through God's commands. In other words, if God command something we know it is right; if God commands against it, we know it is wrong. But what we don't know directly from that is why God commands this or better-worded what about the content might make it so that God commands it and thereby makes it moral or immoral for us.
It turns out most versions of DCT on offer in the philosophical literature are closer in character to the latter than to the former. For these, it could be that God has reasoned that something is immoral on any of a number of bases (Kantian, utilitarian) or that God designed the order of the world to be rational in such a way that some things fit or don't fit.
To wit, when speaking of DCT, we need to be careful to think about whether we are describing a theory wherein our moral knowledge hinges on God's commands or moral reality hinges on God's commands. The latter faces a much larger set of objections than the former. Moreover, both theories could be partial explanations of morality. e.g., the former can include God's commands as a source of moral knowledge rather than the source. Or the metaphysical version can say some rules are arbitrary without declaring all rules arbitrary.
Why all of this legwork? Partially, because the full metaphysical variant has several severe issues (such as murder world above), but the picture in which at least some of morality derives from divine commands seems to be endemic to monotheistic religions.