Revelation 14:1 (ESV)

1 Then I looked, and behold, on Mount Zion stood the Lamb, and with him 144,000 who had his name and his Father’s name written on their foreheads.

The definite article the preceding Lamb specifies that John assumes his readers have remembered the identity of this lamb from his earlier references to it. John had seen a Lamb standing, as though it had been slain (Revelation 5:6). This was clearly Jesus Christ, sacrificed on the altar of Calvary, slain but risen, to whom all authority in heaven and on earth had been given. This is the Lamb John now sees standing even while the dragon and the two beasts from the sea and the earth remain in the background.