See: Michael Pakaluk, Aristotle's Nicomachean Ethics: An Introduction, Cambridge University Press (2005), page 234:
A virtuous person has no desire to do things that are wrong or bad, Aristotle holds, and this is why such a person is pleased by doing what is right, but would be distressed to do something wrong.
Yet we may easily imagine someone who was very much like a virtuous person in what he thinks he should do, but who had persistent desires to do things that he thinks he should not do.
Someone who had certain desires but did not control them, and was prone to act on them, would have akrasia (lack of self-control).
Someone who had such desires, but who typically could control them, not
acting on them, would have enkrateia (self-control).
And see page 238:
Let us say that there is an instance of akrasia when:
(i) a person grasps correctly what he should do (or: his reason says that he should act in some way), but
(ii) he does something else, and
(iii) it is appropriate to say that he does this other thing because of
some impulse or emotion.
Aristotle rejects Socrates' rejection of akrasia:
Aristotle rejects the way Socrates frames the problem, because for Aristotle the difficulty is not whether knowledge might be overpowered, but (roughly) whether human reason might be overpowered by an emotion. [...] Thus, the problem of akrasia, for Aristotle, is to describe a condition in which someone acts irrationally, and does what an emotion provokes rather than what his reason indicates, but where the emotion does not ‘‘overpower’’ his reason, nor does his reason accede to the emotion.
The problem of akrasia for Aristotle, then, is as follows: reason has control over the emotions; something that has control can come to lack it only if either something else takes away its control (it is ‘‘overpowered’’) or it relinquishes control; but an emotion cannot take control away from reason; nor could reason relinquish control to an emotion, except by acceding to what the emotion aims at, in which case there would no longer be any important distinction between akrasia and vice.
And page 254:
as Aristotle sees it, in the central case of akrasia, sensual desire is for a pleasure that would be (as we would say) morally wrong to enjoy.