The Hebrew verb aman (אָמַן) means to be firm, sure, established, or faithful. In the Hiphil stem, it means 'to believe' or 'to trust.' It is the root of the words emunah (H530, faithfulness), emet (H571, truth), and the universal liturgical word Amen.
Aman is the biblical foundation of faith. When Abraham 'believed' God in Genesis 15:6, the Hebrew is the Hiphil of aman — he counted God as firm, reliable, trustworthy. Paul quotes this verse as the cornerstone of justification by faith in Romans 4:3 and Galatians 3:6. The word encapsulates the whole of Israel's relationship with God: to live by faith is to treat God as utterly reliable. The derivative Amen — spoken in worship across all traditions — is literally an affirmation: 'This is firm! This is true! So let it be!'