Both terms are correct. However, often people who convert to Islam often call this a "reversion" as opposed to a "conversion", because of the understanding that Islam is simply the extension of our innate human nature (fitrah).
Because of this, these converts feel that they were always "Muslim", but due to the influence of their society or environment, their ideas and viewpoints differed from those expressed in Islam. But upon studying the teachings of Islam, they feel this unique resonance between their innate nature and Islamic ideology, and thus call their choice to accept Islam a "reversion" to their true nature.
The holy Qur'an speaks about this when it says:
So set your purpose for religion as a man upright (hanifan) by nature (fitrah) - the nature (fitrah) framed by Allah, in which He created mankind. There is no altering (the Laws of) the creation of God. [The holy Qur'an - 30:30]
source : sibtayn
