One could give you the text book chapter long definition. But odds are your looking for some thing concise and with meaning. Yet the answer is one that is defined by the individual, not the group. Ironic, but true. To one it's some one to share a cup of coffee every morning in the corner cafe'. And another it might be their favorite teddy bear that keeps them feeling loved in the dark night. It's up to you find the meaning of companionship in your life.