Interesting question. One comment I agree with from above is:
"Religion is a tool to control the masses."
I would define religion as any system that a person engages in to please/get close to God. Every religion give's people a prescription for this.
Budhism- meditate for multiple lifetimes to acheive enlightenment.
Judaism- obey the Mosaic law.
Islam- obey the teaching of the Quran, which seems to involve praying five times a day and other rules.
Christianity doesn't fir into this definition of religion because Christians do not have to "do" anything to please God and get close to Him. A Christian is defined as a person who believes Jesus is the Son of God and died for their sins. A Christian relies on Jesus' perfect obedience to justify them. A Christian doesn't need to behave, sing to, give money to, or in any way work to appease God because Jesus has done it all for us.
Religion is a system of hoops that some people choose to jump through because they think it will please God. It is a total waste of time because Jesus freely gives us the gift of His perfecty relationship with God as soon as we believe.