It is a common very ancient word, and not a technical term, as it is today (even in Modern Greek).
According to the Liddell, Scott, Jones Ancient Greek Lexicon, ἐπιστήμη
is a word used many-many centuries before Aristotle, deriving from the verb ἐπίσταμαι, (a favorite of Homer!), associated with knowing for certain, capability, skill, expertise, assurance, etc.
From the formal definition:
A.acquaintance with a matter, understanding, skill, as in archery, S.Ph.1057; in war, Th.1.121, 6.72, 7.62; “ἐ. πρὸς τὸν πόλεμον” Lys.33.7 (fort. leg. περί）“ περὶ τὰ μαθήματα” Pl. Phlb.55d; “τοῦ νεῖν” Id.Grg.511c; ἐπιστήμῃ skilfully, οἱ μὴ ἐ. τοὺς ἐπαίνους “ποιούμενοι” Plot.5.5.13.
2.. professional skill: hence, profession, “οἱ τὴν ἰατρικὴν ἐ. μεταχειριζόμενοι” PFay.106.22 (ii A.D.); ζωγράφος τὴν ἐ. painter by profession, POxy.896.5 (iv A.D.).
II.. generally, knowledge, “ἐπιστήμῃ σύ μου προὔχοις ἄν” S.OT1115; πάντ᾽ ἐπιστήμης πλέως full of knowledge in all things, Id.Ant.721, cf. Tr. 338; ἐκ τῆς ἐ. E.Fr.522.3; ἐ. δοξαστική, opp. ἀλήθεια, Pl.Sph.233c: pl., kinds of knowledge, “μυρίαι ἀνδρῶν ἐπιστᾶμαι πέλονται” B.9.38, cf. Pl.Smp.208a.
2.. scientific knowledge, science, opp. δόξα, Pl.R.477b sq., Hp.Lex4, Pl.Plt.301b, Arist.AP0.88b30, EN1139b18: coupled with ἐμπειρία and τέχνη, Pl.R.422c, cf. Ion536c, Arist.Metaph.981a2: pl., the sciences, freq. in Pl.（R.522c,al.), etc.
So it was used by Homer, Thucydides, Sophocles, Plato, Hippocrates, etc, the first easily four centuries before Aristotle. Aristotle used it with a narrow philosophical slant; but the word went on being used all the way to Byzantium (Anna Comnene), with the broad meaning, not the narrow application for "science", as today.
(NB. The verb comes roughly from standing on, mastering, analogous to OE forstandan, German verstehen. I'm not expert enough to trace understand... under, not on? Go figure...)