"Ecclesia" is the Greek word in the Bible that is usually rendered in English as "church." But "church" isn't really a translation of "ecclesia." The English translation of "ecclesia" is "assembly," in the sense of a legislative body such as the Senate or the House of Representatives.

The English word "church", which is substituted for "assembly" when the Greek word "ecclesia" is used, comes from a different Greek word, "kyriakon."

"Kyriakon" is a possessive adjective, meaning "the master's"  (belonging to the head of a household) or "the Lord's" (belonging to the ultimate head of household). It began to be applied to buildings for worship around 300 AD, in the phrase, "Kyriakon doma," "the house of the Lord."

As of 3/19/11, I don't know how translators came to replace the English term for a group of people ("assembly") with the term for a sacred building ("church"). But they did.