Question: What does Pentecost mean?
Answer: The English word “Pentecost” is a transliteration of the Greek word pentekostos, which means “fifty.” It comes from the ancient Christian expression pentekoste hemera, which means “fiftieth day.”
Originally Pentecost was used by Greek speaking Jews to refer to the Jewish Festival of Weeks (Shavuot) which was celebrated fifty days after Passover.
During the Jewish celebration of Shavuot/Pentecost (fifty days after Jesus's resurrection), the Holy Spirit was poured out upon his first followers, thus empowering them for their mission and gathering them together as a church. So, in essence this was the beginning of the church
For more reading:
http://www.patheos.com/blogs/markdroberts/series/what-is-pentecost-why-does-it-matter/