Lamb of God is one of the titles given to Jesus Christ in the New Testament and consequently in the Christian tradition.

The title is found just twice in the New Testament, in the Gospel according to John:

The next day he [John the Baptist] saw Jesus coming toward him and said, "Behold, the Lamb of God, who takes away the sin of the world." (John 1:29)

The next day John was there again with two of his disciples, and as he watched Jesus walk by, he said, "Behold, the Lamb of God." (John 1:35f)

But the image of Christ as lamb is very prominent in Revelation, where Christ 28 times is referred to as Lamb and also appears in Acts 8:32, 1 Cor 5:7 (implied) and 1 Peter 1:19.

The title has to be understood against the background of:

1. the paschal lamb of the Old Testament whose blood protected and saved the Israelites (Exodus 12). This link is explicit in 1 Cor 5:7. For Paul, Christians are saved by Christ as their true paschal lamb.
2. the Old Testament practice of sin offerings. Lambs could be used in these offerings (e.g. Lev 4:32-34 and 5:6). This link is strongly suggested by John 1:29 and 1 Peter 1:19. Like the sin of a person could be forgiven through the offering and the pouring out of the blood of an "unblemished" lamb (cf. Lev 4:32) so Christians would be freed from sin by the blood of Christ as unblemished lamb.
3. the suffering servant of Isaiah 53 who "like a lamb led to the slaughter" (53:7) was silent and "gives his life as an offering for sin" (53:10). This link is explicit in Acts 8:32 and strengthens the idea of Christ as sin offering.

Revelation uses another Greek word for Lamb and so the apocalyptic picture of a ruling and victorious lamb probably should not be read into the title "Lamb of God" in John.

In heraldry, a lamb of God (or paschal lamb, or agnus Dei) is a lamb passant proper, with a halo Or charged with a cross gules, and the dexter forelimb reflexed over a cross staff from which a pennon of St. George (Argent a cross gules) is flotant.