IIRC: The unforgivable sin is the absolute denial of God’s love and forgiveness.
Here again, I deffer to James Akin:
(** I can not stress enough that you follow the link below and read this entire entry in the context. Mr. Akin has a great deal to say about this topic and this little quote doesn’t nearly cover it! ** - as usual, any bold or colouring in the following text is by my hand

)
www.ewtn.com - THE UNFORGIVABLE SIN - James Akin
(…) in the preceding verse, Jesus asserts (v 30) that one must ally with him or be opposed to him and “through this” he tells us (v 31) that the blasphemy against the Spirit will not be forgiven.
Blaspheming the Spirit is thus a failure to repent and ally oneself with Jesus. Since this can always be done during one’s life (cf. 20:1-15), blasphemy against the Holy Spirit must be a final refusal to repent, or final impenitence.
Thus the official stand of the Catholic Church’s, following Augustine and a whole host of subsequent moral theologians, is that the blasphemy against the Holy Spirit is final impenitence. [Saint] Pope John Paul II writes:
“Against the background of what has been said so far, certain other words of Jesus, shocking and disturbing ones, become easier to understand. . . . They are reported for us by the Synoptics in connection with a particular sin which is called ‘blasphemy against the Holy Spirit.’ . . . Why is blasphemy against the Holy Spirit unforgivable? How should this blasphemy be understood? Saint Thomas Aquinas replies that it is a question of a sin that is ‘unforgivable by its very nature, insofar as it excludes the elements through which the forgiveness of sin takes place’ (ST 2b:14:3).
According to such an exegesis, ‘blasphemy’ does not properly consist in offending against the Holy Spirit in words; it consists rather in the refusal to accept the salvation which God offers to man through the Holy Spirit, working through the power of the Cross. If man rejects the ‘convincing concerning sin’ which comes from the Holy Spirit and which has the power to save, he also rejects the ‘coming’ of the Counsellor . . . If Jesus says that blasphemy against the Holy Spirit cannot be forgiven either in this life or in the next, it is because this ‘non-forgiveness’ is linked, as to its cause, to ‘non-repentance’, in other words to the radical refusal to be converted. . . .
Blasphemy against the Holy Spirit, then, is the sin committed by the person who claims to have a ‘right’ to persist in evil—in any sin at all . . . [T]he Church constantly implores with the greatest fervor that there will be no increase in the world of the sin that the Gospel calls ‘blasphemy against the Holy Spirit.’ Rather, she prays that it will decrease in human souls” (Encyclical Letter Dominum et Vivificantem “The Lord and Giver of Life”] 46-47).
(…)