The Greek noun doule is the feminine form of doulos (slave/servant). It is used by Mary in her magnificent response to the angel Gabriel, and by Luke when quoting Joel's prophecy.
Doule reaches its theological apex in Luke 1:38, where Mary says, 'I am the Lord's servant (doule). May your word to me be fulfilled.' This is one of the most profound declarations of faith in all of Scripture — complete surrender to God's will without full understanding of the consequences. Mary's self-identification as God's slave-girl echoes the Old Testament anawim (the humble poor who trust God). Joel 2:29 prophesies that God will pour out His Spirit on male and female servants alike — fulfilled at Pentecost.