Mysticism is the knowledge of, and especially the personal experience of, states of consciousness, i.e. levels of being, beyond normal human perception, including experience and even communion with a supreme being.
A "mystikos" was an initiate of a mystery religion

Greco-Roman mysteries

Mystery religions, sacred Mysteries or simply mysteries, were religious cults of the Greco-Roman world, participation in which was reserved to initiates....

. The Eleusinian Mysteries

Eleusinian Mysteries

The Eleusinian Mysteries were initiation ceremonies held every year for the cult of Demeter and Persephone based at Eleusis in ancient Greece. Of all the mysteries celebrated in ancient times, these were held to be the ones of greatest importance...

, (Greek: Ἐλευσίνια Μυστήρια) were annual initiation ceremonies in the cults of the goddesses Demeter

Demeter

In Greek mythology, Demeter is the goddess of the harvest, who presided over grains, the fertility of the earth, and the seasons . Her common surnames are Sito as the giver of food or corn/grain and Thesmophoros as a mark of the civilized existence of agricultural society...

 and Persephone

Persephone

In Greek mythology, Persephone , also called Kore , is the daughter of Zeus and the harvest-goddess Demeter, and queen of the underworld; she was abducted by Hades, the god-king of the underworld....

, held in secret at Eleusis (near Athens

Athens

Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...

) in ancient Greece

Ancient Greece

Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

.