The act or offense of speaking sacrilegiously about God or sacred things; profane talk.
In other words, the act or offense of speaking about religion as though you are not religious. Speaking about a religion as if you are not an adherent of it. And all of us are at least non-adherents of all religions except our own. Some of us aren't adherents of any religions.
Therefore we are all blasphemers.
Most of us try not to gratuitously insult the religious beliefs of others. This is considered a gesture of respect for the person, since religious beliefs and behavior are not regarded as ordinary beliefs and behavior, but as part of a person's identity. Perhaps the most important part, to them. But belonging to an exclusivist religion means believing that other religions are not paths to God-- at least, not as direct paths as yours is. So even if they don't say so, adherents of these faiths believe that other faiths are wrong. Or at least mistaken. If you are a committed skeptic, you are aware that religions generally make empirical claims, and some of those empirical claims are false. They do not align with objective reality, so far as you can tell. And if you are an ethical and honest person, you recognize and are willing to acknowledge that sometimes adherents of religions commit grossly harmful acts, and that sometimes they even exalt as admirable figures people who have committed grossly harmful acts in the name of their deity or deities.
Therefore if you are an adherent of an exclusivist faith, a skeptic, and/or an ethical and honest person, you are a blasphemer.
And yet in some places in the world, blasphemy is either illegal or on its way to becoming so. In other places in the world it isn't illegal, but people consider it grounds to physically attack someone. If you condemn the latter but approve of the former, you are like Shaykh Abdallah bin Bayyah, Vice Chairman of the International Union of Muslim Scholars who recently cautioned fellow Muslims to refuse to respond to depictions of Muhammad, even insulting ones, with violence. That was admirable, but in the same breath he also asked the U.N. and Western governments to make it criminal to "denigrate the religious symbols" of Muslims. As commenter Abby Normal eloquently put it, "He essentially wants to replace chaotic mob violence with orderly state violence." It is not the job of the mob or the state to commit violence in order to protect religious feelings.
For these reasons I celebrate International Blasphemy Rights Day today. Not because I get a thrill in provoking or antagonizing, but because I recognize that doing so is both inevitable and necessary. And that religious feelings, while special to those who have them, cannot dictate the freedom of others to speak. If you want to join me in celebrating this day, you don't need to blaspheme if you don't want to (or at least, you don't have to knowingly blaspheme, though you very likely will on accident). You can just think about it. And maybe tell someone else, so they will think about it. That in itself will benefit us all.