Mulhid ( , plural ) is a person who rejects the existence of God ( ) and disbelieves in the Last Day, also known as an Atheist. A Mulhid is considered a Kafir because of that person's Kufr (Disbelief).
The Qur'an refers to Atheists in this verse:
And they say, "There is not but our worldly life; we die and live, and nothing destroys us except time." And they have of that no knowledge; they are only assuming. [Al-Jāthiyah 45:24]