Used often when discussing religion to specify belief in just one god, compared to polytheist. She's a monotheist, worshipping only one deity, unlike ancient Greek polytheists.
Pro Tip 2/3
Cultural Sensitivity
When discussing beliefs, use 'monotheist' respectfully to acknowledge someone's faith. He's a devout monotheist and takes his beliefs very seriously.
Pro Tip 3/3
Not Interchangeable
Don't confuse 'monotheist' with 'monogamous'. The first is about god-beliefs, the second about relationships. They are monotheists in faith, but discussing their personal lives would involve the term 'monogamous'.