The New International Dictionary of New Testament Theology defines ekklesia as "assembly, meeting, congregation, church." It says of the word's classical usage:
ekklesia, derived via ek-kaleo, which was used for the summons to the army to assemble, from kaleo, to call. It is attested from Eur. and Hdt. onwards (5th cent. B.C.), and denotes in the usage of antiquity the popular assembly of the competent full citizens of the polis, city.
An example of this secular usage is found in Acts 19 (the bold highlighted words translate ekklesia):
About that time there arose a great disturbance about the Way. A silversmith named Demetrius, who made silver shrines of Artemis, brought in no little business for the craftsmen. He called them together, along with the workmen in related trades, and said: "Men, you know we receive a good income from this business. And you see and hear how this fellow Paul has convinced and led astray large numbers of people here in Ephesus and in practically the whole province of Asia. He says that man-made gods are no gods at all. There is danger not only that our trade will lose its good name, but also that the temple of the great goddess Artemis will be discredited, and the goddess herself, who is worshiped throughout the province of Asia and the world, will be robbed of her divine majesty."
When they heard this, they were furious and began shouting: "Great is Artemis of the Ephesians!" Soon the whole city was in an uproar. The people seized Gaius and Aristarchus, Paul's traveling companions from Macedonia, and rushed as one man into the theater. Paul wanted to appear before the crowd, but the disciples would not let him. Even some of the officials of the province, friends of Paul, sent him a message begging him not to venture into the theater.
The assembly was in confusion: Some were shouting one thing, some another. Most of the people did not even know why they were there. The Jews pushed Alexander to the front, and some of the crowd shouted instructions to him. He motioned for silence in order to make a defense before the people. But when they realized he was a Jew, they all shouted in unison for about two hours: "Great is Artemis of the Ephesians!"
The city clerk quieted the crowd and said: "Men of Ephesus, doesn't all the world know that the city of Ephesus is the guardian of the temple of the great Artemis and of her image, which fell from heaven? Therefore, since these facts are undeniable, you ought to be quiet and not do anything rash. You have brought these men here, though they have neither robbed temples nor blasphemed our goddess. If, then, Demetrius and his fellow craftsmen have a grievance against anybody, the courts are open and there are proconsuls. They can press charges. If there is anything further you want to bring up, it must be settled in a legal assembly. As it is, we are in danger of being charged with rioting because of today's events. In that case we would not be able to account for this commotion, since there is no reason for it." After he had said this, he dismissed the assembly (Acts 19:23-41).
Some (e.g.) have argued that all instances of ekklesia in the New Testament should be translated by "assembly" or "gathering."
But there is also a long history of referring to the church as "the called out" ones, based on the etymology of ekklesia. Some (e.g.) reference D. A. Carson's book Exegetical Fallacies to caution against this practice. Carson writes:
One of the most enduring fallacies, the root fallacy presupposes that every word actually has a meaning bound up with its shape or its components. In this view, meaning is determined by etymology; that is by the roots of a word.
Carson's warning is important. Usage is more important than etymology. However, we shouldn't just look at secular usage; we should also look at Old Testament usage. In the case of ekklesia, Carson writes in his commentary on Matthew in The Expositor's Bible Commentary:
Whenever ekklesia in the LXX is translating Hebrew, the Hebrew word is qahal ("assembly," "meeting," "gathering"), with reference to various kinds of "assemblies" ... but increasingly used to refer to God's people, the assembly of Yahweh.
The point about ekklesia as "God's people, the assembly of Yahweh," is important. It seems to me that simply speaking of ekklesia as an assembly or gathering can lead to the idea that the church is mainly about meeting together. But ekklesia is not so much the activity as it is the people - the community or fellowship, if you will - who have been assembled or gathered together by God. On a side note, aren't there "religious" overtones to this, not in the sense of an institution, but in the sense of a community of faith? So I think there is still benefit in thinking of the church as "called out" ones - or those "called together" by God. Indeed, Paul can speak of the community of God's people as the kletoi, the called ones (e.g. Romans 1:6).
Just as there are those who think they can be part of a church without ever meeting, there are others who think that the meeting is church. Both are distortions. The ekklesia is the people of God. And we gather together because we've been called together in the name of Jesus Christ.