Ephesians 2:8-9 New International Version (NIV)

For it is by grace you have been saved, through faith—
and this is not from yourselves,
it is the gift of God not by works, so that no one can boast.
 
 
Grace in Christianity is the free and unmerited favour of God as manifested in the salvation of sinners and the bestowing of blessings.
Common Christian teaching is that grace is unmerited mercy (favor) that God gave to humanity by sending his Son, Jesus Christ, to die on a cross, thus securing man’s eternal salvation from sin. 
 
Definition from Wikipedia