I’ll list them from least complex and bare to most complex and descriptive, but all three are fairly short.
The church is that collection of people who are hearing the Word of God, responding to it with their lives, and who have obeyed Jesus’ specific commands to be baptized and proclaim his death in the Lord’s Supper.
Elsewhere Dever wrote (“The Church” in A Theology for the Church, 768):
The church is the body of people called by God’s grace through faith in Christ to glorify him by serving him in his world.
Me (PJ) (heavily dependent on believers.org quoted below):
The church is a group of Christians associated together under Christ to unite individuals to Christ and itself and to take responsibility for one another’s discipleship.
The local church is a community of regenerated believers who confess Jesus Christ as Lord. In obedience to Scripture they organize under qualified leadership, gather regularly for preaching and worship, observe the biblical sacraments of baptism and communion, are unified by the Spirit, are disciplined for holiness, and scatter to fulfill the great commandment and the great commission as missionaries to the world for God’s glory and their joy.
Do you have any thoughts on this? I just reflected on the fact that the way you define church shapes the way you approach church planting.
Update:
The church is the gospel people who believe the gospel, who are a formed community around the gospel, and who demonstrate the gospel in everyday ways of life so that the world might know that the Father sent the Son.
Jeff Vanderstelt just posted another definition of the church with a good intro:
The Church is God’s People (who we are) saved by God’s Power (what He has done and is doing) for God’s Purposes (the good works he created us in Jesus Christ to do).
Art Azurdia:
A local church is a group of Christians who have intentionally united together for the expressed purpose of exercising New Testament Christianity to the glory of God and on behalf of one another in the world.
Believers.org defines a church like this:
A church is a group of believers in Jesus Christ, associated together, under Christ, for His purposes.
A local church is a group of Christians who regularly gather in Christ’s name to officially affirm and oversee one another’s membership in Jesus Christ and his kingdom through gospel preaching and gospel ordinances.
A local church is constituted by a group of Christians gathering together bearing Christ’s own authority to exercise the power of the keys of binding and loosing.
A local church is “a group of Christians who regularly gather to proclaim the gospel (around a confession), affirm one another by the ordinances (around a constitution), live together as a family of God (around a covenant).”
The local church is a group of people who are united with Christ through faith in the gospel of Jesus Christ and repentance of sins and who have been united with one another by Christ’s baptism of them into his body by the Holy Spirit. These individuals have then obeyed Christ in receiving the outward physical sign of the basis for the forgiveness of their sins (the death and resurrection of Jesus) and their baptism by the Spirit, namely water baptism by immersion. A local church is led by pastors, served by deacons, administers the ordinances (baptism and the Lord’s Supper) and is governed congregationally under the headship of Jesus Christ by His Word. Local churches should seek to exposit the Scriptures in preaching, exalt the Savior in worship, equip the saints in discipleship, and evangelize sinners as their mission.
A local church is a manifestation in time and space of the ultimate, heavenly-eschatological assembly of God’s redeemed humanity. Each local church represents what it means to be God’s people by assembling together in one place for worship, encouragement, and accountability; by being a community shaped by the Scriptures; by observing the symbol-laden acts of baptism and the Lord’s supper; by maintaining its purity through church discipline; and by seeking to make disciples of all nations. All of this is to be done under the leadership of elders, the service of deacons, and the rule of the congregation.
PJ's personal blog. For articles see gospelize.me 
By including church discipline and not including baptism and the Lord’s Supper it seems as if you are saying church discipline is more fundamental to a church being a church than the Sacraments/ordinances. Do you mean to say this?
By not including regenerate church membership in your definition are you merely recognizing that there will (most likely) be non-regenerate church members in a given local church, or are you saying that to be a member of a local church one need not be regenerate? Or maybe neither of those thoughts is correct. Regenerate church membership does seem implicit in your definition of a church, especially if you include “have faith in” in “are committed to” when referring to the Lord Jesus, but it is not explicit.
This is good to think through PJ. MD has a great definition! I find it hard to believe his emphasis on “qualified leadership” and “disciplined for holiness” though. BTW, you got up pretty early this morning! -AC
Caleb – I’m not saying church discipline is more fundamental to a church then baptism and the Lord’s Supper. Well, I might be saying that. I’m not sure. What I do know I’m saying is that fundamental to the church is (saving) belief in the gospel, holy love for one another in a church community, and mission together of making disciples. I think fundamental in love for one another is church discipline because it distinguishes between say my love for you and my love for Ben Bratcher. I love you, but I can’t love you with a holy love to the point of church discipline because our level of commitment to each other is not at that church-defining level. So it defines the lines of who is in a local church and who is not. If I just say love for each other, then 2 Christians like us may be a church since we’re both committed to the gospel and both committed to the mission.
Baptism and the Lord’s Supper are inherent in mission. There is no direct universal command in Scripture to the unbeliever to be baptized. There is only the command to disciples to baptize the disciples they make (Matt. 28:20). But baptism can happen outside of the local church too (Acts 8). But if you as a church are making disciples of the Lord Jesus, then they must be baptized and must observe the Lord’s supper among other commands (Matt. 28:20). I understand the historical Protestant reason for elevating the sacraments in the definition of the church to distinguish from Roman Catholic practices of the sacraments, but my definition is meant to be a biblical-theological definition apart from historical discussion, as valid and necessary as that discussion is.
And yes, implicit in believing the gospel and being committed to Jesus Christ is regeneration because I don’t believe you can truly believe and commit to Christ and the gospel apart from the regenerative work of the Holy Spirit. So my definition does not fit well with our infant-baptist brothers.
PJ said: “And yes, implicit in believing the gospel and being committed to Jesus Christ is regeneration…”
But your definition doesn’t say “believing”, it merely says “committed”. That’s where the disconnect could be, not for me because I know where you are coming from, but for others who merely hear that that is what a church is.
Church – is an organized congregation of born-again scripturally baptized individual in a given place, meeting in covenant with God & with each other to carry out the spirit & letter of Christ’s commission as found in Matthew 28:18-20
A group of people gathered in Christ who are collectively and personally responsible for one another’s discipleship toward Christ Jesus in order to disciple their neighbors and the nations.