Much of the standard Christian religious terminology -- and Holy Ghost/Spirit in particular -- derives from the New Testament. This is a collection of 27 books, all written in Ancient Greek, which form the latter portion of the Christian Bible. Most of us don't read Ancient Greek these days, so we use translations of the New Testament books. And translation from one language to another can often involve tricky issues.

The particular phrase found in the New Testament that is rendered in English translations as "Holy Spirit" or "Holy Ghost" is "πνεῦμα ἅγιον" (pneuma hagion) -- or a variant of this, depending on the sometimes wacky requirements of Greek grammar. The word order is reversed from the usual order in English: pneuma is spirit/ghost, and hagion is holy.

Translating pneuma into English can be problematic. The word is the ordinary Ancient Greek word for "wind" or "breath". Coming out of that, the word is also used in the New Testament to mean "the thing that makes you alive": if you are breathing, then you are alive; otherwise, you are dead. Thus, where appropriate, we translate pneuma as "spirit".

Now, what about "ghost"? There is an older meaning of the English word "ghost", little used today, that is more along the lines of "spirit". See, for example, the Wiktionary entry for "ghost", English noun definition 1, which is noted to be "dated".

But this meaning of "ghost" was quite current in 1599, when the Geneva Bible (an English translation) was published, and in 1611, when the King James Version (another English translation) was published. The phrase pneuma hagion occurs in the New Testament over 80 times, and both of these translations nearly always render it in English as "Holy Ghost", each using the term "Holy Spirit" only 4 times in the New Testament.

But languages change, and that meaning of "ghost" faded with time. By the beginning of the 20th century, the English word "ghost" meant essentially what it does today: an active, incorporeal remnant of a dead person. So translating pneuma hagion as "Holy Ghost" no longer conveyed the appropriate meaning to a speaker of English. And, indeed, in the American Standard Version, an English translation of the Bible published in 1901, the phrase "Holy Ghost" never appears at all, having been entirely replaced by "Holy Spirit". The vast majority of more recent Bible translations are similar.

Nonetheless, the term "Holy Ghost" continued to be used by many Christian groups throughout the 20th century, and it is still used by some today. The main reason for this is the continued use of older Bible translations -- primarily the King James Version. This translation is over 400 years old, but many people still read it, and, as noted above, it mostly says, "Holy Ghost". A related reason is sticking with tradition; if you've always said "Holy Ghost", then you are likely to continue doing so.

In short:

• "Holy Ghost" and "Holy Spirit" are English renderings of the same Ancient Greek phrase, and are intended to mean the same thing.

• There was no particular point in time when everyone switched. Rather, use of "Holy Ghost" has faded, and use of "Holy Spirit" has increased, since the beginning of the 20th century.

• The reason for the change is that the English language changed, with "ghost" no longer being used the way it used to be. But the change was delayed and slowed by the continued use of older Bible translations.

As for some particular church changing its name or the terminology it uses: they do that whenever they feel like, for their own reasons. It is possible that the Roman Catholic Church decreed a change in English terminology at some point, but I cannot say when that might have been.