Merriam-Webster.com

Definition of Pentecost

 1: SHAVUOT: a Jewish holiday observed on the sixth of Sivan and by Orthodox and Conservative Jews in the Diaspora also on the seventh of Sivan in commemoration of the revelation of the Ten Commandments at Mt. Sinai

— called also Pentecost

2: a Christian feast on the seventh Sunday after Easter commemorating the descent of the Holy Spirit on the apostles

— called also Whitsunday

Etymonline.com

Old English Pentecosten “Christian festival on seventh Sunday after Easter,” from Late Latin pentecoste, from Greek pentekostē (hēmera) “fiftieth (day),” fem. of pentekostos, from pentekonta “fifty,” from pente “five” (from PIE root *penkwe- “five”). The Hellenic name for the Old Testament Feast of Weeks, a Jewish harvest festival observed on 50th day of the Omer (see Leviticus xxiii.16).

Discussion/Explanation

It is a word and title with multiple contexts.

Besides the above, Pentecost is also at the center of Protestants who consider themselves Pentecostal. They are named so because of their focus on the descent of the Holy Spirit. There is variance within the denomination but they all fixate on there being an expression of the Holy Spirit’s presence in a person’s conversion to following Christ. The common expression of the Holy Spirit in this context is speaking in tongues. At least some Pentecostals even make speaking in tongues a requirement to be considered truly saved.