Plethos (πλῆθος) means a multitude, large number, crowd, or assembly. From plethyo (to be full), it describes a large quantity of people or things gathered together.
Plethos appears throughout Luke-Acts to describe the growing early church. In Acts 2:6, a multitude gathers at Pentecost. In Acts 4:32, the multitude of believers is of one heart and mind. Luke uses this word to show explosive numerical growth — a fulfillment of God's promise to Abraham. The plethos of the saved in Revelation 7:9 — a great multitude no one could count — is the ultimate fulfillment. James 5:20 speaks of covering a multitude of sins. The repeated appearance of plethos encourages us: the gospel is not for the few but for the many.