While citta seems to show some variations according to context, it's literal meaning is "thought", but also has been identified primarily with "state of mind". Here's Sue Hamilton analysis1:
By this I mean that citta is the term used to refer to the qualitative picture, as it were, of the way all one's mental processes are functioning at any given moment. This is very different from any of the different mental processes of which the whole 'mind' is comprised: citta is neither an entity nor a process (which probably accounts for its not being classified as a khandha, nor mentioned in the paticcasamuppada formula), but is a term which abstractly indicates one's progress on the path, or even more generally, one's mental condition.
Understanding citta to mean 'state of mind' clarifies otherwise confusing passages, such as the following examples: "Without understanding the thoughts of his mind, one is reborn in life after life [...] with a restless state of mind". Likewise: "My state of mind is not of such nature that it will return to kamabhava levels of existence; knowing this, one's state of mind is well acquainted with wisdom". Johansson suggests these (and other similar) passages imply that citta
"seems to signify a surviving entity". But what they are indicating is that one's state of mind reflects one's progress, or lack of it, on the path to insight.
Claiming that citta is everlasting is not just a challenge to suññatā, but a challenge to anatta, and thus, a contradiction to pretty much the entire canon on that matter (whose summary has been written as sabbe dhamma anatta: all dhammas are not-self). Finally:
"Bhikkhus, you may well cling to that doctrine of self that would not arouse sorrow, lamentation, pain, grief, and despair in one who clings to it. But do you see any such doctrine of self, bhikkhus?"
—"No, venerable sir."
—"Good, bhikkhus. I too do not see any doctrine of self that would not arouse sorrow, lamentation, pain, grief, and despair in one who clings to it."
-- MN 22
1 See "Identity and Experience: The Constitution of the Human Being According to Early Buddhism"