What does the word EASTER mean?
Question: What does the word Easter mean in Acts 12:4?
Answer: First, let us take a look at the King James Bible verse in question in its context.
The second part of your question needs a bit of clarification and explanation. Biblically speaking, the Passover was not a "Jewish" festival although Jews (members of the tribe of Judah or citizens of the Kingdom of Judah) certainly celebrated this Feast day. The Passover is one of God's annual Holy Days he commanded ancient Israel to keep.
The first century church did not rename the Passover to Easter. The word is a derivative of Ishtar or Astarte. It is the name of a pagan goddess of fertility celebrated during the Spring solstice.
The introduction of Sunday as a day to commemorate the death of Christ first began after Roman Emperor Hadrian crushed a Jewish rebellion (132-135 A.D.). After his victory he rebuilt the city ruins and expelled all Jews and Christians. He undertook a policy of banning the practice of any religious ceremony or event that resembled what the Jews practiced (e.g. Saturday Sabbath, etc.).
As the church historian Eusebius states, a replacement occurred of Jerusalem's Jewish-Christian leaders with Gentile church members and leaders. The new leaders changed the traditional date of observing the Christian Passover to SUNDAY. Many Gentile-Christian churches such as the one found in Rome adopted such changes. Over time, the name of the celebration changed.
The confusion concerning the word in the Bible began because scholars whose orientation was toward Catholic and Anglican church doctrines wrote the King James Version Bible. This bias caused some errors in translating the original Bible text into English. One such error was translating the Greek word pascha (Strong's Concordance #G3957), which means "Passover," as the English word Easter.