Alan Hirsch makes this excellent post concerning the definition of church. I'm impressed with the entire thing, but especially the second point down at the bottom of the post.
Alan says that a definition of church must include this activity: "Cultivate covenant community." Hey, three consecutive occurences of "c" in a row! Let's unpack it for just a second.
Cultivate. This is what you do in the garden to help your plants grow. You rake and hoe around the plants to kill weeds, and to keep the soil stirred up and soft which helps watering be more effective and allows nutrients to get more directly to where they need to be. It's putting forth effort toward the good of the plant.
Covenant. This is a promise. It's the nature of relationship in the church, because our common denominator is Jesus, and our relationship to him. We have in common our promise to him, and that includes promises to care for each other, to be involved in each other's lives.
Community. This is actually being together. Most people are involved in multiple communities. Work. School. Friends with whom they stay in regular contact. Church. Small group (Sunday School is one).