From a prolonged form of suneido; co-perception, i.e. Moral consciousness -- conscience.

see GREEK suneido

a. the consciousness of anything: with a genitive of the object, τῶν ἁμαρτιῶν, a soul conscious of sins, Hebrews 10:2 (τοῦ μύσους, Diodorus 4, 65; συνείδησις εὐγενής, consciousness of nobility; a soul mindful of its noble origin, Herodian, 7, 1, 8 (3 edition, Bekker)).

b. "the soul as distinguishing between what is morally good and bad, prompting to do the former and shun the latter, commending the one, condemning the other; conscience": with a genitive of the subjunctive, ἡ συνείδησις τίνος, Romans 2:15 (where the idea of ἡ συνείδησις is further explained by καί μεταξύ ... ἡ καί ἀπολογουμένων (cf. Winers Grammar, 580 (539); see ἀπολογέομαι, 2, and συμμαρτυρέω)); Romans 9:1; 1 Corinthians 8:7 (cf. Winer's Grammar, § 30, 1 a.),; ; 2 Corinthians 1:12; 2 Corinthians 4:2; 2 Corinthians 5:11; Hebrews 9:14 (ἡ τοῦ φαυλου συνείδησις, Philo, fragment, vol. ii, p. 659, Mangey edition (vi., p. 217f, Richter edition)); ἡ ἰδίᾳ συνείδησις, 1 Timothy 4:2; ἄλλη συνείδησις equivalent to ἄλλου τίνος συνείδησις, 1 Corinthians 10:29; διά τήν συνείδησιν, for conscience' sake, because conscience requires it (viz., the conduct in question), Romans 13:5; in order not to occasion scruples of conscience (in another), 1 Corinthians 10:28; μηδέν ἀνακρίνειν διά τήν συνείδησιν (anxiously) questioning nothing, as though such questioning were demanded by conscience, 1 Corinthians 10:25, 27; διά συνείδησιν Θεοῦ, because conscience is impressed and governed by the idea of God (and so understands that griefs are to be borne according to God's will), 1 Peter 2:19; ἡ συνείδησιν τοῦ εἰδώλου, a conscience impressed and controlled by an idea of the idol (i. e. by a notion of the idol's existence and power), 1 Corinthians 8:7 Rec.; τελειῶσαι τινα κατά τήν συνείδησιν (namely, αὐτοῦ), so to perfect one that his own conscience is satisfied, i. e. that he can regard himself as free from guilt, Hebrews 9:9; ἐλέγχεσθαι ὑπό τῆς συνειδήσεως John 8:9 (ὑπό τοῦ συνειδοτος, Philo de Josepho § 9 at the end; συνέχεσθαι τῇ συνειδήσει, Wis. 17:10); ἡ συνείδησις is said μαρτυρεῖν, Romans 9:1; συμμαρτύρειν, Romans 2:15; τό μαρτύριον τῆς συνειδήσεως, 2 Corinthians 1:12. With epithets: ἀσθενής, not strong enough to distinguish clearly between things lawful for a Christian and things unlawful, 1 Corinthians 8:7, cf. 1 Corinthians 8:10; συνείδησις ἀγαθή, a conscience reconciled to God, 1 Peter 3:21; free from guilt, consciousness of rectitude, of right conduct, Acts 23:1; 1 Timothy 1:5 (Herodian, 6, 3, 9 (4 edition, Bekker)); ἔχειν συνείδησιν ἀγαθήν, 1 Timothy 1:19; 1 Peter 3:16, (ἐν ἀγαθή συνείδησις ὑπάρχειν, Clement of Rome, 1 Cor. 41, 1 [ET]); ἔχειν συνείδησιν καλήν, Hebrews 13:18; συνείδησις καθαρά, 1 Timothy 3:9; 2 Timothy 1:3 (Clement of Rome, 1 Cor. 45, 7 [ET], cf. ἁγνή συνείδησις, ibid. 1, 3; καθαρός τῇ συνειδήσει, Ignatius ad Trall. 7, 2 [ET]); ἀπρόσκοπος, Acts 24:16; πονηρά, a mind conscious of wrong-doing, Hebrews 10:22 ((ἐν συνειδήσει ποιηρα, 'Teaching' etc. 4, 14 [ET]); ἀπρεπής, Lucian, amor. 49). ἡ συνείδησις καθαρίζεται ἀπό κτλ., Hebrews 9:14; μολύνεται, 1 Corinthians 8:7; μιαίνεται, Titus 1:15 (μηδέν ἑκουσίως ψεύδεσθαι μηδέ μιαίνειν τήν αὑτοῦ συνείδησιν, Dionysius Halicarnassus, jud. Thucydides8ἅπασιν ἡμῖν ἡ συνείδησις Θεός, Menander 597, p. 103, Didot edition; βροτοῖς ἅπασιν ἡ συνείδησις Θεός, ibid. 654, p. 101, Didot edition; Epictetus fragment 97 represents ἡ συνείδησις as filling the same office in adults which a tutor (παιδαγωγός, which see) holds toward boys; with Philo, Plutarch, and others, τό συνειδός is more common. In the Sept. once for מַדָּע , Ecclesiastes 10:20; (equivalent to conscience, Wis. 17:11; cf. Delitzsch, Brief an d. Röm., p. 11)). Cf. especially Jahnel, Diss. de conscientiae notione, qualis fuerit apud veteres et apud Christianos usque ad aevi medii exitum. Berol. 1862 (also the same, Ueber den Begr. Gewissen in d. Griech. Philos. (Berlin, 1872)); Kähler, Das Gewissen. I. die Entwickelung seiner Namen u. seines Begriffes. i., Alterth. u. N. T. (Halle, 1878); (also in Herzog edition 2, under the word Gewissen; Zezschwitz, Profangräcität as above with, pp. 52-57; Schenkel, under the word Gewissen both in Herzog edition 1, and in his BL.; P. Ewald, De vocis συνείδησις ap. script. Novi Test. vi ac potestate (pp. 91; 1883); other references in Schaff-Herzog, under the word Conscience).