Quick Definition
ὄρθρος, ου, ὁ n.m. early dawn, day-break
Strong's Definition
from the same as 3735; dawn (as sun-rise, rising of light); by extension, morn
KJV Usage: early in the morning
UBS Dictionary of the Greek New Testament
noun, m. | -ου | ὀρθρίζω
1 There may be slight differences of meaning between αὐγή and ὄρθρος[b] in that αὐγή focuses primarily upon light and ὄρθρος[b] perhaps on the early part of dawn, but this cannot be determined precisely from existing contexts.
UBS Dictionary of the Greek New Testament © United Bible Societies, licensed CC BY-SA 4.0.
Thayer's Greek Lexicon
ὄρθρος, -ov, 6, (fr. OPQ, ὄρνυμε to stir up, rouse; ef. Lat. orior, ortus), fr. Hes. down; Sept. for >nw dawn, and several times for 193; daybreak, dawn: ὄρθρου Baθέος or βαθέως (see βαθέως and βαθύς [on the gen. οἵ. W. § 30,11; B. § 132, 26]), at early dawn, Lk. xxiv. 1; ὄρθρου, at daybreak, at dawn, early in the morning, Jn. viii. 2 (Hes. opp.575; Sept. Jer. xxv. 45 xxxiii. (xxvi.) 5, etc.) ; ὑπὸ τὸν ὄρθρον, Acts v. 21 (Dio Cass. 76, 17).
Mounce Concise Greek Dictionary
ὄρθρος orthros 3x
the dawn; the morning, Jn. 8:2; Acts 5:21; ὄρθος βαθύς, the first streak of dawn, the early dawn, Lk. 24:1*
Abbott-Smith Greek Lexicon
ὄρθρος, -ου, ὁ,
[in LXX for שַׁחַר, בֹּקֶר, etc.;]
daybreak, dawn: ὄρθρου βαθέως (M, Pr., 73), at early dawn, Lk 24:1; ὄρθρου, Jo 8:[2]; ὑπὸ τὸν ὄ., Ac 5:21 (cf. MM, xviii).†
Liddell-Scott-Jones — Greek-English Lexicon
ὄρθρος (orthros) — dawn. ὄρθρος, ὁ, the time just before or about daybreak, dawn, cock-crow (ἀπ᾽ ὄρθρου μέχρι περ ἂν ἥλιος ἀνάσχῃ [Refs 5th c.BC+]at dawn, [Refs 8th c.BC+]; πρὸς ὄρθρον towards dawn, [Refs 5th c.BC+]; τὸν ὄ., absolutely, in the morning, [Refs 5th c.BC+]; δι᾽ ὄρθρων each morning early, [Refs 5th c.BC+]; ὄ. βαθύς dim morning twilight, ἀλλὰ νῦν ὄ. β. [NT+5th c.BC+]
II. Ὄρθρος, ὁ, a mythical dog, son of Typhaon and Echidna, that kept the herds of Geryoneus on the island Erytheia, and was there killed by Heracles, [Refs 8th c.BC+]