Abarim Publications' online Biblical Greek Dictionary
The noun δενδρον (dendron) means tree and stems from the ancient Proto-Indo-European root "deru-", meaning to be firm, solid or steadfast (hence also English words like dendrite, Druid, durable, duress, endure, during, and so on). The Hebrew equivalent of this Greek word is עץ ('es), tree, which relates to עצם ('osem), meaning either might or skeleton.
Our noun δενδρον (dendron) tends to denote a living and natural tree, whereas the noun ξυλον (xulon) mostly describes chopped or worked wood, or else a domesticated and cultivated tree.
The running theme of the tree in the Bible — from the Tree of Life and the Tree of Knowledge of Good and Evil in Paradise to the gopher-wooden ark of Noah, Abraham's terebinths at Mamre (the word for oak, namely אלה, 'allah, is obviously similar to the word for God: Eloah or אלה), the cedars of Tyre from which Solomon built the Temple of YHWH, to ultimately the cross of Christ and the Tree of Life of the restored earth (Revelation 22:2) — are rather obviously not about trees but about schools of thought and traditions: stations in mankind's quest for wisdom, or bus stops on the journey from the caves to the New Jerusalem.
See our article on the noun ριζα (riza), meaning root, for a quick look at how trees and forests relate to the human mind. Read our article on the noun αμπελος (ampelos), vine, for a look at how the united mind of man is self-similar to a vineyard.
Our noun δενδρον (dendron), tree, is used 26 times in the New Testament; see full concordance.