a person or thing acting or serving in place of another.
    to put (a person or thing) in the place of another.

The heart of the message of the cross is substitution. Jesus took our place. This is reinforced again and again in Isaiah 53:

    v4 surely he has borne our griefs and carried our sorrows
    v5 he was wounded for our transgressions
    v5 he was crushed for our iniquities
    v5 upon him was the chastisement that brought us peace
    v5 with his stripes we are healed
    v6 the Lord has laid on him the iniquity of us all
    v8 he was cut off out of the land of the living, stricken for the transgression of my people
    v11 he shall bear their iniquities
    v12 he bore the sin of many
"The concept of substitution may be said, then, to lie at the heart of both sin and salvation. For the essence of sin is man substituting himself for God, while the essence of salvation is God substituting himself for man. Man asserts himself against God and mputs himself where only God deserves to be; God sacrifices himself for man and puts himself where only man deserves to be. Man claims prerogatives which belong to God alone; God accepts penalties which belong to man alone." John Stott, The Cross of Christ