What is the Unforgivable Sin?

Blasphemy is the only unforgivable sin.

Definition of Blasphemy:  (Merriam-Webster)
     1. the act of insulting or showing contempt or lack of reverence for God.
     2. irreverence toward something considered sacred or inviolable.

       
However, the scripture below tells us that blasphemy against God or Jesus Christ can still be forgiven, it's only blasphemy against the Holy Spirit. 

Matthew 12:31-32  "And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man (Jesus) will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."