From en and meno; to stay in the same place, i.e. (figuratively) persevere -- continue.
see GREEK en
see GREEK meno
a. properly, in a place: ἐν τίνι, Acts 28:30 T Tr WH.
b. to persevere in anything, a state of mind, etc.; to hold fast, be true to, abide by, keep: τῇ πίστει, Acts 14:22 (νόμῳ, ὅρκοις, etc. in the Greek writings); ἐν τίνι (more rarely so in the classics, as ἐν ταῖς σπονδαις, Thucydides 4, 118; ἐν τῇ πίστει, Polybius 3, 70, 4): ἐν (so R G only) τοῖς γεγραμμένοις, Galatians 3:10 from Deuteronomy 27:26; ἐν τῇ διαθήκη, Hebrews 8:9 from Jeremiah 38:32 STRONGS NT 1696a: ἐμμέσῳἐμμέσῳ, equivalent to ἐν μέσῳ (see μέσος, 2): Revelation 1:13; Revelation 2:1; Revelation 4:6; Revelation 5:6; Revelation 22:2, in Tdf. edition 7; (see his Proleg., p. xlviii. (but nowhere in edition 8, see the Proleg., p. 76f); cf. WHs Appendix, p. 150; B. 8).