Apparently a primary word (compare "door"); a portal or entrance (the opening or the closure, literally or figuratively) -- door, gate.
a. properly: κλείειν, etc. τήν θύραν, Matthew 6:6; Luke 13:25; passive, Matthew 25:10; Luke 11:7; John 20:19, 26; Acts 21:30; ἀνοίγειν, Acts 5:19; passive Acts 16:26f; κρούειν, Acts 12:13; διά τῆς θυρης, John 10:1f; πρός τήν θύραν, Mark 1:33; Mark 11:4 (Tr WH omit τήν; cf. Winer's Grammar, 123 (116)); Acts 3:2; τά πρός τήν θύραν the vestibule (so Buttmann, § 125, 9; others the space or parts at (near) the door), Mark 2:2; πρός τῇ θύρα John 18:16; ἐπί τῇ θύρα, Acts 5:9; πρό τῆς θύρας, Acts 12:6; ἐπί τῶν θυρῶν, Acts 5:23 (R G πρό).
b. θύρα is used of any opening like a door, an entrance, way or passage into: ἡ θύρα τοῦ μνημείου, of the tomb, Matthew 27:60; Matthew 28:2 R G; Mark 15:46; Mark 16:3, (Homer, Odyssey 9, 243; 12, 256; others).
c. in parable and metaphorically, we find α. ἡ θύρα τῶν προβάτων, the door through which the sheep go out and in, the name of him who brings salvation to those who follow his guidance, John 10:7, 9; cf. Christ. From Fritzsche in Fritzschiorum opuscc., p. 20ff; (in Ignatius ad Philad. 9 [ET] Christ is called ἡ θύρα τοῦ πατρός, δἰ ἧς ἐισερχονται Ἀβραάμ ... καί οἱ προφῆται; cf. Harnack on Clement of Rome, 1 Cor. 48, 3f [ET]). β. 'an open door' is used of the opportunity of doing something: τῆς πίστεως, of getting faith, Acts 14:27; open to a teacher, i. e. the opportunity of teaching others, 2 Corinthians 2:12; Colossians 4:3; by a bold combination of metaphor and literal language, the phrase θύρα μεγάλη καί ἐνεργής (A. V. a great door and effectual) is used of a large opportunity of teaching a great multitude the way of salvation, and one encouraging the hope of the most successful results: 1 Corinthians 16:9. γ. the door of the kingdom of heaven (likened to a palace) denotes the conditions which must be complied with in order to be received into the kingdom of God: Luke 13:24 (for Rec. πύλης); power of entering, access into, God's eternal kingdom, Revelation 3:8 cf. Revelation 3:7 (but others besides; add here Revelation 4:1). δ. he whose advent is just at hand is said ἐπί θύραις εἶναι, Matthew 24:33; Mark 13:29, and πρό θυρῶν ἑστηκεναι, James 5:9. ε. ἑστηκώς ἐπί τήν θύραν καί κρούων is said of Christ seeking entrance into souls, and they who comply with his entreaty are said ἀνοίγειν τήν θύραν, Revelation 3:20.