Abarim Publications' online Biblical Greek Dictionary
The noun θυρα (thura) means door. It stems from an ancient proto-Indo-European root "dhwer-" from which also stems our English word door. It's officially not clear what this root actually expresses (which verb it relates to) but the words for door that derive from it often occur in dual forms, which has made some people believe that the ancients had double doors. Here at Abarim Publications we rather doubt that, also because these words stem from a time when houses had no actual doors but curtains — perhaps indeed double ones so that the actual slit ran down the middle, but the dual or plural forms in which our word so often occurs is probably because dwellings normally came with a multitude of openings (entrances, windows, smoke ports; all wide open or with curtains first and doors or shutters later).
The Hebrew words for door, namely דל (dal) and דלת (delet), relate to the verbs דלה (dala) and דלל (dalal), which mean to hang, which would be explained by the typical nature of old-world doors, namely that of hanging curtains. But from our PIE root "dhwer-" also come the word forest and foreign, which demonstrate that our word θυρα (thura) much rather signified a point of transition across the boundary between within and without. When PIE was a living language, houses were as novel as hard drives are today, and doors were doubtlessly regarded with the same sense of marvel as houses were themselves. When privacy and social stratification were invented, and single "houses" contained entire extended families, the houses that made this possible tied into the deepest definitions of what it meant to be human. Hence in Hebrew the words for house (בית, bayit) and for daughter (בת, bat) look surprisingly alike, and so do the words for son (בן, ben), the verb to build (בנה, bana) and the word for stone (אבן, 'eben).
The walls of a house demonstrated who was in and who wasn't and the door was the instrument of transition. Our word much rather describes the hole than the cover of it, which is demonstrated by its use in describing the "door" of the tomb (Matthew 27:60). When Jesus spoke of a door behind which one should pray (Matthew 6:6), he not so much emphasized one's privacy (because that would be better obtained in the wilderness or mountains) but rather the family one declared himself part of. Conversely, when he observed that "it" would open when knocked upon (Matthew 7:7), he probably didn't remind his audience of a warm welcoming haven behind the door but the war that would come pouring out of Quirinus' war doors if the Jews wouldn't stop revolting.
Also see our article on the Hebrew root ספף (sapap), which likewise deals with the corridors of transition between the within and the without.
Our noun is used 39 times, see full concordance, and from it derive:
- The noun θυρεος (thureos), literally a door-thing: a door stop that would keep the door from opening and thus allow for undesired traffic between the outside world and the inside. In later Greek this noun came to describe the kind of long, half-cylindrical door-like shield of the Romans (who called it a scutum), and that's probably the meaning Paul employed (it occurs in Ephesians 6:16 only). Crucially in this context, the traditional round shield had become replaced by the scutum when the Roman army no longer fought traditionally, in an every-man-for-himself sort of way, but as interlocked maniples. Paul's famous "shield of faith" is not the attribute of the lone warier, but one of a disciplined and well-trained formation.
- The noun θυρις (thuris), which is a diminutive form of the parent noun and literally means little door. It's the common word for window, which in Biblical times was indeed much rather a door, without glass and possibly with a shutter, through which items were exchanged (Acts 20:9 and 2 Corinthians 11:33 only).
- Together with the otherwise unused noun ουρος (ouros), watcher or guardian: the noun θυρωρος (thuroros), meaning door-keeper or porter. This noun is used 4 times; see full concordance; twice in its feminine form, which tells us that the doorkeeper who let Peter in was a lady (John 18:16-17).