The Greek noun arrabōn (a Semitic loanword, related to Hebrew erabon) refers to a deposit, down payment, pledge, or earnest — a portion of the full amount given in advance as a guarantee that the rest will follow. In modern Greek, the word still means 'engagement ring.' Paul uses this commercial term as a rich theological metaphor for the Holy Spirit.
In all three New Testament occurrences (2 Corinthians 1:22; 5:5; Ephesians 1:14), arrabōn describes the Holy Spirit as God's deposit — the first installment of the full inheritance yet to come. This is extraordinarily significant: the Spirit is not merely a gift but a guarantee. The down payment commits the giver to deliver the full amount. God has given believers the Spirit as His irrevocable pledge that the full redemption of their bodies and the complete inheritance of glory are certain. The Spirit in the believer is God's own signature on the contract of salvation.