dryad n.

also *dreu-, Proto-Indo-European root meaning "be firm, solid, steadfast," with specialized senses "wood," "tree" and derivatives referring to objects made of wood.

It forms all or part of: betroth; Dante; dendrite; dendro-; dendrochronology; dour; Druid; drupe; dryad; dura mater; durable; durance; duration; duress; during; durum; endure; hamadryad; indurate; obdurate; perdurable; philodendron; rhododendron; shelter; tar (n.1) "viscous liquid;" tray; tree; trig (adj.) "smart, trim;" trim; troth; trough; trow; truce; true; trust; truth; tryst.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dru "tree, wood," daru "wood, log, timber;" Greek drys "oak," drymos "copse, thicket," doru "beam, shaft of a spear;" Old Church Slavonic drievo "tree, wood," Serbian drvo "tree," drva "wood," Russian drevo "tree, wood," Czech drva, Polish drwa "wood;" Lithuanian drūtas "firm," derva "pine, wood;" Welsh drud, Old Irish dron "strong," Welsh derw "true," Old Irish derb "sure," Old Irish daur, Welsh derwen "oak;" Albanian drusk "oak;" Old English treo, treow "tree," triewe "faithful, trustworthy, honest."

word-forming element of Greek origin appended to nouns and denoting collective numerals (triad, Olympiad) and fem. patronymics (Dryad, Naiad, also, in plural, Pleiades, Hyades), thence also plant family names; from Greek -as (genitive -ados), fem. suffix equivalent to -is.

From its use in Iliad (literally "of Ilion," that is, "Troy;" from Ilias poiesis or oidos "poem of Ilion," the accompanying noun being feminine, hence the termination) it has formed titles of poems in imitation of it (Columbiad, Dunciad).

evergreen shrub found in cold or Alpine regions in the northern hemisphere, 1798, from Greek dryas (see dryad). As an indicator of tundra climate, the presence of its remains in lake-bed sediments lent its name to the Younger Dryas, the name given to the period of sharp and sudden return to Ice Age conditions in Europe c. 12,000 years ago.