an assembly: a body of people called together and gathered as a group, whether a town's citizens, the people of Israel, or a community of believers in Jesus.
In everyday Greek of the first century this word simply meant a gathering of people summoned to meet. A Greek town had its own regular assembly of citizens that decided public business, and the word could equally name a chance crowd or any group convened for a purpose. There is nothing inherently religious about it. The popular pulpit idea that it literally means "the called-out ones," read off the parts of the word, is rejected by the standard lexicons: by the first century speakers heard "assembly," not a built-in theology of separation from the world.
When Greek-speaking Jews translated the Hebrew Scriptures, they reached for this same word to render the great gathering of Israel before God (קהל), above all the people assembled at the mountain when the covenant was given. So a second layer of meaning rode alongside the civic one: the assembly of God's people. Both layers were live at once, and that double background is exactly what made the word useful when the first Christians needed a name for themselves.
The early believers took the word and used it for their own gatherings:
- the handful who met in someone's house
- the congregation of a whole city
- the believers of a region
- at full stretch the entire people of God as one body belonging to Christ.
The thread running through every use, secular or sacred, small or vast, is the same: a group of people gathered as a single body. The differences between the senses below are differences of who is gathered and on what footing, not different unrelated words.