The term 'pentecostal' generally refers to the spiritual experience that first occured on the Day of Pentecost, a Jewish feast known in Hebrew as Shavuot. This feast took place seven weeks after Passover, which explains the Hebrew name (shavuot means weeks). The feast was actually on the day after the end of the seventh week, which explains the Greek name (pentecost means fifty).
The Bible says, 'When the Day of Pentecost had fully come, they were all with one accord in one place.2 And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting.3 Then there appeared to them divided tongues, as of fire, and one sat upon each of them.4 And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance' (Acts 2:1-4).
This is the Pentecostal experience: speaking in tongues as the Spirit of God gives the utterance. Today, those who have this experience are often called Pentecostals. Pentecost is not a religion in the taditional sense; it is an experience.
For additional information, please read the articles on the Doctrine shelf of the Apostolic Free Library.