ekklesia, usually translated as "church" in most English versions of the Bible, is a compound word meaning "out of" (ek) and "called" (kaleo). Etymologically it means "the called out".
Whereas it's unwise to accept the etymological meaning of a word as its actual meaning ("pineapple" has nothing to do with either pines or apples [except somewhat for it's appearance in the eyes of medieval botanists]), in this case it's pretty safe.
The earliest usage of the word appears to date back to early Greek City-States, and refers to political restructuring, when citizens might be "called out" of a city to form, as a group, a new government structure. A political coup, so to speak.
By New Testament times it was a little less political, a little less revolutionary, sometimes referring to simply a crowd, even a rowdy one, like when the town clerk dismisses the rioting mob in Acts 19:41, using the word ekklesia.
(The "churches of Christ" in Rom 16:16 could be rendered just as accurately as the "mobs of the Messiah".)
When Jesus announced that he would build his church, he was essentially saying that he was going to call out from the world a people of his own, to form a new government, and we see this sort of description both in earlier scripture:
WEB Isa 9:7 Of the increase of his government and of peace there shall be no end, on David’s throne, and on his kingdom, to establish it, and to uphold it with justice and with righteousness from that time on, even forever.and later scripture:
WEB 1 Pet 2:9 But you are a chosen race, a royal priesthood, a holy nation, a people for God’s own possession, that you may proclaim the excellence of him who called you out of darkness into his marvelous light.