Quick Definition
ἰσχύς, ύος, ἡ n.f. strength, power
Strong's Definition
from a derivative of is (force; compare eschon, a form of 2192); forcefulness (literally or figuratively)
KJV Usage: ability, might(-ily), power, strength
UBS Dictionary of the Greek New Testament
noun, f. | -ύος | ἰσχύω
74.8 · Able, Capable
capability, strength exceptional capability, with the probable implication of personal potential In a number of languages the expression ‘to love the Lord with one’s strength,’ in the sense of physical strength, is meaningless, and therefore it may be necessary to translate ‘love the Lord your God as completely as you can.’ It is important to note that in a passage such as MRK 12:30 the terms in the series καρδίαψυχήδιάνοια, and ἰσχύς do not refer to completely different parts or aspects of human personality; rather, the four are combined to emphasize the totality of the individual. In some languages the equivalent way of expressing this comprehensive aspect of personality is ‘to love him completely with all you feel and all you think.’
UBS Dictionary of the Greek New Testament © United Bible Societies, licensed CC BY-SA 4.0.
Thayer's Greek Lexicon
ἰσχύς, vos, ἡ, (ἴσχω [allied w. ἔσχον ; to holdin check), [fr. Hes. down], Sept. esp. for N35, 7H, ΤΡ, 79923; ability, force, strength, might: 2 Pet. ii. 11 (joined w. δύναμις) ; Rey. y. 12; vii. 12; τὸ κράτος τῆς ἰσχύος, power (over external things) afforded by strength, Eph. i. 19; vi. 10, (Is. xl. 26); ἡ δόξα τῆς icy. (see δόξα, III. 3 Ὁ. a. fin.), 2Th.i. 9; κράζειν ἐν ἰσχύει, with strength, mightily, Rev. xviii. 2 Rec.; ἐξ ἰσχύος, of one’s strength, to the extent of one’s ability, 1 Pet. iv. 11; with ὅλης added, Mk. xii. 30, 33; Lk. x. 27 [here L txt. T Tr WH read ἐν ὅλῃ τῇ ἰσχύϊ]. [Syn. see δύναμις, fin.]
Mounce Concise Greek Dictionary
ἰσχύς ischus 10x
strength, might, power, Rev. 18:2; Eph. 1:19; faculty, ability, 1 Pet. 4:11; Mk. 12:30, 33; Lk. 10:27
Abbott-Smith Greek Lexicon
ἰσχύς, -ύος, ἡ,
[in LXX chiefly for כֹּחַ, also for חַיִל, עֹז, גְּבוּרָה, etc.;]
strength, might, power, force, ability: Re 5:12 7:12; ἰ. κ. δύναμις, II Pe 2:11; τ. κράτος τῆς ἰ. (Is 40:26), Eph 1:19 6:10; ἡ δόξα τῆς ἰ., II Th 1:9; ἐξ ἰ., I Pe 4:11; ἐν ὅλῃ τ. ἰ., Mk 12:30, 33(LXX), Lk 10:27(LXX).†
SYN.: δύναμις (q.v., and cf. ICC, Phl 4:13); κράτος (cf. M, Th., I, 1:9).
Moulton & Milligan — Vocabulary of the Greek NT
ἰσχύς. The only exx. of this subst. from our sources are late, e.g. P Lond 13195 (A.D. 544 or 545) (=III. p. 272) τὴν ἰδίαν ἰσχὺν καὶ δύναμιν, and BGU II. 371”? (Arab.) πράσεως τῆς . ἐχόυσης τὴν ἰδίαν πίστειν (/.—1v) kal ἰσχὺν εἰς πλήρης | καὶ εἰς ὁλόκληρον, with reference to the validity of a receipt.
Liddell-Scott-Jones — Greek-English Lexicon
ἰσχύς (ischus) — strength. ἰσχ-ύς [see. entry near the end], ύος, ἡ, strength of body, [Refs 8th c.BC+]; of places, ἰσχὺς γῆς [Refs 5th c.BC+]; of a fortified place, [Refs 5th c.BC+]
2. might, power, θεοῦ, θεῶν, [Refs 5th c.BC+]; ὅπου γὰρ ἰ. συζυγοῦσι καὶ δίκη might and right, [Refs]; φύσεως ἰ., of Themistocles, [Refs 5th c.BC+]; ἐπὶ μέγα ἐλθεῖν ἰσχύος to a great height of power, [Refs]; ἰ. μάχης fighting power, [Refs]; ἡ τῶν νόμων ἰ. [Refs 4th c.AD+]; validity, [Refs 3rd c.AD+]
3. brute force, κατ᾽ ἰσχύν perforce, opposed to δόλῳ, [Refs 5th c.BC+]
4. motive force, [Refs 4th c.BC+]; ἡ κινοῦσα ἰ. [Refs]
5. in Lit. Crit., vigour of style, [Refs 1st c.BC+]
II. in Tactics, the main body of troops, οὔπω ἡ ἰ. πάρεστιν [Refs 5th c.BC+]. [ῠ in genitive, etc.: in nominative and accusative singular ῠ in [Refs 5th c.BC+]: ῡ in Trag. and Comedy texts, [Refs 5th c.BC+]