What is the unforgivable sin?
Jesus was clear, the unforgivable sin is slandering the Holy Spirit, and saying it is something it is not. In the particular case where it is mentioned in the Bible, it was reducing the works of the Holy spirit to that of an "impure spirit", "the Devil". It's saying The works of the Holy Spirit such as healing, raising people from the dead, and so on, are of the powers of evil, not the power of good. Slander, simply, or lying about what the Holy Spirit is.
The following is exactly what the bible says it is, and that is not even close to some of the definitions given here. If I were some of you, I'd be very careful, because if we define it wrongly people will not know what it is, and they may commit it, and maybe because of what you told, or didn't tell them. Please...stop listening to preachers who's teachings do not line up with the Bible.
Here, see for yourselves exactly what it is:
Mark 3:20-30 NIV
20 Then Jesus entered a house, and again a crowd gathered, so that he and his disciples were not even able to eat.
21 When his family heard about this, they went to take charge of him, for they said, “He is out of his mind.”
22 And the teachers of the law who came down from Jerusalem said, “He is possessed by Beelzebul! By the prince of demons he is driving out demons.”
23 So Jesus called them over to him and began to speak to them in parables: “How can Satan drive out Satan?
24 If a kingdom is divided against itself, that kingdom cannot stand.
25 If a house is divided against itself, that house cannot stand.
26 And if Satan opposes himself and is divided, he cannot stand; his end has come.
27 In fact, no one can enter a strong man’s house without first tying him up. Then he can plunder the strong man’s house.
28 Truly I tell you, people can be forgiven all their sins and every slander they utter,
29 but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.”
30 He said this because they were saying, “He has an impure spirit.”