thumb|262px|Men reacting enthusiastically
In modern usage, "enthusiasm" refers to intense enjoyment, interest, or approval expressed by a person. The term is allegedly related to playfulness, inventiveness, optimism, zest, verve, and high energy.
People can often characterise the modern version of enthusiasm as "infectious" or "contagious",
and sometimes even more metaphorically as a virus in its own right.
The word once applied, in accordance with its etymology,
to persons possessed by a god, or to people who exhibited intense piety.
Historical usage
thumb|Methodist preachers have been known for their enthusiasm in promulgating the doctrines of the [[Born again#Methodism|new birth and entire sanctification to the public at events such as tent revivals and camp meetings, which they believe is the reason that God raised them up into existence.]]
The word enthusiasm originates from the Greek from (, 'in') and (, 'god'), meaning 'inspired or possessed by [a] god'. Applied by the Greeks to manifestations of divine possession, by Apollo (as in the case of the Pythia), or by Dionysus (as in the case of the Bacchantes and Maenads), the term enthusiasm was also used in a transferred or figurative sense. Socrates taught that the inspiration of poets is a form of enthusiasm. The term was confined to a belief in religious inspiration, or to intense religious fervor or emotion.
From this, a Syrian sect of the fourth century was known as the Enthusiasts. They believed that "by perpetual prayer, ascetic practices, and contemplation, man could become inspired by the Holy Spirit, in spite of the ruling evil spirit, which the fall had given to him". From their belief in the efficacy of prayer, they were also known as Euchites.
Several Protestant Christian denominations that emerged in the 16th and 17th centuries, especially those who hold revivals, were called enthusiastic.
During the 18th century, popular Methodists such as John Wesley or George Whitefield were accused of blind enthusiasm, a charge against which they defended themselves by distinguishing fanaticism from "religion of the heart." Methodists who enthusiastically preach about and experience the new birth (first work of grace) and entire sanctification (second work of grace) often have emotional experiences.