Little contemporary evidence about druids exists, and thus little can be said regarding them with assurance. It is known that they held the cultural repository of knowledge in an oral tradition, using poetic verse as a mnemonic device and to ensure the fidelity of the transmission of knowledge over time. Most of what is known about them comes from the Roman writers. Similar to the monks of the Christian era following, they combined the duties of priest, judge, scholar, and teacher. The core points of druidic doctrine known to us from Roman sources is their belief in metempsychosis, and their practice of human sacrifice. However, reports of atrocities used to justify imperial conquest are to be considered critically throughout history. Their reported reverence for various aspects of the natural world, such as the ritual of oak and mistletoe described by Pliny the Elder, has also been associated with animism.Mac Mathúna, Liam (1999) "Irish Perceptions of the Cosmos" Celtica vol. 23 (1999), pp.174-187

The earliest record of the name druidae (lang: Δρυΐδαι) is reported from a lost work of the Greek doxographer Sotion of Alexandria (early second 2nd BCE), who was cited by Diogenes Laertius in the 3rd century CE.

Modern attempts at reconstructing, reinventing or reimagining the practices of the druids in the wake of Celtic revivalism are known as Neo-druidism.

Etymology

The English word druid derives from Latin druides ( ), which is the same as the term used by Greek ethnographers, lang: δρυίδης (druidēs). The Latin and Greek terms are loans from a Proto-Celtic stem *druwid-, which combines the Proto-Indo-European roots *deru- and *weid-.

The word was etymologized (as per Aristides) as containing δρύς "oak tree"), and the Greek suffix -ιδης. *deru- is indeed the Indo-European "oak" word (cognate to English tree), but the root has a wider array of meanings related to "to be firm, solid, steadfast" (whence e.g. English true), and it isn't clear whether the term was originally derived from a meaning involving "oak", or the wider meaning of "true, solid".

*weid- is the Indo-European root for "to see" and, by extension and figurative use, also referred to knowledge, as in English wit, wisdom, Latin vision or Sanskrit veda.

The Old Celtic (Gaulish) term from which the Greek and Latin druides was derived has survived in its Insular Celtic form, in Old Irish druídecht (IPA2: ˈdriːðʲext), which yields Modern Irish draoiocht ( ), "magic." The Welsh dryw (IPA2: ), "seer", may be cognate. The Modern Irish for druid is drúa 1,Fact: date=October 2008 from Old Irish druí 2; which also produced Irish draoi 3, "magician" and Modern Gaelic druidh 4, meaning "enchanter" and draoidh 5, "magician."