Concise Bible Dictionary:

The divinely appointed system of Jewish ordinances which formed the enclosure into which the Lord entered by the door, in order to find His own sheep and lead them out. Gentile believers were added to them, and they became one flock (not “one fold”) with one Shepherd, the Lord Himself (John 10:1, 3, 16). There is no longer a fold on earth for those that are Christ's. They are formed into the church, namely, the one flock.
Sheepfold—Taken at Beit Sahur, December 4, 1937