An Islamic word in Arabic for "hypocrite."
A munafiq is much worse than an ordinary hypocrite, you see. His or her external appearance is that they follow Islam. If you ask whether they are a muslim, they will reply yes. People will see them pray and follow Islamic activities like fasting, but inside, they don't believe. They may go to the mosque, but its just for show, so people will see them. Many don't even realize that they lack faith.
Some do this because they are uncertain about whether God really exists. Others do this because the mainstream of society follows the religion, and they play along. Most are Muslim in name only.
Muhammad once said "A munafiq is he who lies, breaks promises, and proves himself dishonest if trusted."
The Qur'an has this to say about them:
Of the people there are some who say: "We believe in God and Judgment Day;" but they are not believers. They try to deceive God and those who believe, but they only deceive themselves, and don't realize it! In their hearts is a disease; and God has increased their disease: And grievous is the penalty they (incur), because they are false (to themselves). When it is said to them: "Make not mischief on the earth," they say: "Why, we only Want to make peace!" Of a surety, they are the ones who make mischief, but they realise (it) not. (2:8-12)
See also the eleven verse chapter of the Qur'an, Surah 63 The Hypocrites
One hadith Sahih Al-Bukhari quotes the Prophet (pbuh) as saying, "Whoever has (the following) four characters will be a hypocrite, and whoever has one of the following four characteristics will have one characteristic of hypocrisy until he gives it up. These are: (1 ) Whenever he talks, he tells a lie; (2) whenever he makes a promise, he breaks it; (3) whenever he makes a covenant he proves treacherous; (4) and whenever he quarrels, he behaves impudently in an evil insulting manner."
A munafiq is much more dangerous and worse than a disbeliever. In the early days of Islam, some ill-intentioned people pretended to convert, then used the opportunity to argue with religious leaders and stir up dissension amongst the Muslim population. If they weren't a Muslim, they would be ignored, but a person who professes to be Muslim is trusted. He or she is a risk because they can lead followers astray by defying the rules.
Hypocrites are therefore the worst kind of liar. "When thou lookest at them, their exteriors please thee; and when they speak, thou listenest to their words. They are as (worthless as hollow) pieces of timber propped up, (unable to stand on their own). They think that every cry is against them. They are the enemies; so beware of them. The curse of God be on them! How are they deluded (away from the Truth)!" (63:4)
Many people have been accused of hypocrisy, and I'm sure it's a major challenge to someone to be called that. You shouldn't refer to someone as a munafiq, since you don't know what's in their heart, only they and God know.
Saddam Hussein fits the description of a munafiq; he claims to be a good practicing Muslim and even lets reporters videotape him in prayer, but not-so-secretly he breaks his own rules by getting drunk frequently and having mistresses. I can point out that what he does wrong is sinful, but it's not up to me to say he doesn't believe in God, so I'm not supposed to accuse him of being a munafiq.