Adoption is the act of God whereby He gives the believing sinner his position, standing, legal rights, inheritance as a joint-heir with Christ, and all the full privileges of an adult son/(daughter) of God. 

Adoption is contrasted with regeneration. Regeneration is the new birth that changes us and imparts unto us the holy nature of our Father and makes us a child of God. Adoption does not change our nature, but rather it changes our position and legal standing as adult sons (daughters) of God and thus guarantees us an inheritance.