Do you know who coined the word "passover" in English? Do you know what the Anglo/Saxon/German word "ostern" means? Tyndale, loved to use multiple words in English, etc to translate from one Hebrew, Syriac or Greek word.
The words are defined by scripture, not by man's present use.
'Easter' (shortened form of 'easterlamb' 1 Cor. 5:7, and a carry over use/translation from Tyndale, Matthew, Genevans, Luther) is passover. You might find this article helpful, though I do not necesarily agree with everything in it -
Riches of Grace: Easter
There is no mystery here. Simply trace the word in any concordance, and let the Bible define the word. It isn't hard, or complicated to the serious and prayerful student of God's word.