From didomi (G1325, "to give"), dorea denotes a gift — and crucially, a gift given freely without expectation of return or payment. It is distinguished from other "gift" words by its emphasis on gracious, unearned bestowal. Paul's concentrated use of dorea in Romans 5 to describe justification and the Spirit emphasizes that salvation is gift through and through — nothing earned, nothing merited.
Dorea is the grammar of grace. Every great blessing of the gospel is described with this word: the gift of righteousness (Romans 5:17), the gift of the Holy Spirit (Acts 2:38; John 4:10), God's indescribable gift (2 Corinthians 9:15). Simon the Sorcerer's attempt to purchase dorea with money represents every religious effort to earn what can only be received (Acts 8:20). The entire economy of grace runs on dorea: given freely by a lavish God, received by empty hands of faith.