The feminine noun ἀπάντησις occurs three times in the GNT: Matt. 25:6; Acts 28:15; 1 Thess. 4:17. Each occurrence is also in the accusative case.
William Mounce provides this definition: "a meeting, encounter, εἰς ἀπάντησιν to meet"
1 Thessalonians 4:17 (SBLGNT): ἔπειτα ἡμεῖς οἱ ζῶντες οἱ περιλειπόμενοι ἅμα σὺν αὐτοῖς ἁρπαγησόμεθα ἐν νεφέλαις εἰς ἀπάντησιν τοῦ κυρίου εἰς ἀέρα· καὶ οὕτως πάντοτε σὺν κυρίῳ ἐσόμεθα.
Rogers and Rogers: ἀπάντησις meeting. The word had a technical meaning in the Hellenistic world related to the visits of dignitaries to cities where the visitor would be formally met by the citizens, or a deputation of them, who went out from the city and would then ceremonially escort him back into the city (Best).
Jeffrey A.D. Weima (Baker Exegetical Series): "Although not all of these elements of a Hellenistic formal reception parallel the events surrounding the return of Christ described in 1 Thess. 4:15–17 (see the six points raised by Cosby 1994: 28–31), the differences can be readily explained (see the response to Cosby by Gundry 1996); thus it is highly probable that Paul’s use of apantēsis reflects this civic custom in antiquity (so most commentators). This conclusion is all the more convincing in light of the preceding reference to Christ’s return with the term parousia. As Green (2002: 227–28) declares: 'Since the context of this formal reception is the time of the royal parousia of the Lord (v. 15), there remains little doubt that this custom formed the background of this teaching.'"
See Colin R. Nicoll, From Hope to Despair in Thessalonica: Situating 1 and 2 Thessalonians. Cambridge: Cambridge University Press, 2004. Pages 45-48.
Compare Robert H. Gundry. “A Brief Note on “Hellenistic Formal Receptions and Paul’s Use of ΑΠΑΝΤΗΣΙΣ in 1 Thessalonians 4:17.” Bulletin for Biblical Research, Vol. 6 (1996): 39-41.