Lexical Definition

‎ἰδού‎ idoú (id-oo') look! G:INJ
second person singular imperative middle voice of ‎εἴδω‎

--behold, lo, see.

Usage: 4 forms in 1267 verses

Abbott-Smith

ἰδού, [in LXX chiefly for הִנֵּה,] prop. imperat. 2 aor. mid. of ὁράω, used as a demonstrative particle, with frequency much greater in LXX and NT than in cl. (v. M, Pr., 11),
lo, behold, see: Mt 10:16 11:8 13:3, Mk 3:32, Lk 2:48, I Co 15:51, Ja 5:9, Ju 14, Re 1:7, al.; after gen. absol., Mt 1:20 2:1, 13 12:46, al.; καὶ ἰδού, Mt 2:9 (and freq.), Lk 1:20 10:25, Ac 12:7, al.; in elliptical sentences, taking the place of copula or predicate (like הִנֵּה in Heb.), Mt 3:17, Lk 5:12 22:31, 47, Ac 8:27, 36 al.

Abbott-Smith References

Book Refs
Matthew 1:20 · 2:1 · 2:9 · 2:13 · 3:17 · 10:16 · 11:8 · 12:46 · 13:3
Mark 3:32
Luke 1:20 · 2:48 · 5:12 · 10:25 · 22:31 · 22:47
Acts 8:27 · 8:36 · 12:7
1 Corinthians 15:51
James 5:9
Jude 1:14
Revelation 1:7

Liddell-Scott-Jones

ἰδού

(ἴδου Hdn.Gr. cited in [Refs 4th c.AD+]aorist 2 imperative middle of ὁρά; but,

__II ἰδού (on the accent see Hdn.Gr.[Refs] adverb, lo! behold! (even with words of hearing, ἰδοὺ δοῦπον αὖ κλύω τινά [Refs 5th c.BC+]
__II.1 with Nouns and Prons., . χελιδών Klein [Refs 6th c.BC+], etc.; . ἐγώ here am I, [LXX+NT]; οὐκ . Ἀαρώ; [LXX]
__II.2 with Verbs,
__II.2.a in the imperative, . θεᾶσθε [Refs 5th c.BC+]; especially in offering a thing, take it! . δέχου παῖ [Refs 5th c.BC+]
__II.2.b in indicative of all tenses, . πείθομαι [NT+5th c.BC+] with past tenses, [LXX+NT]; in the middle of a sentence, [NT]
__II.3 with questions, ., τί ἔστι; [Refs 5th c.BC+]
__II.4 in repeating another's words quizzingly, as ἰδού γ᾽ ἄκρατον wine, quotha! [Refs]; . λέγειν[Refs]

LSJ Scripture Refs: LXX.Gen.27.1Luke.19.20LXX.Exod.4.14LXX.Gen.24.15Matt.27.51Luke.13.16