Religion

During our many discussions you and I have both used the term "religion". Accoding to Marx it is the "Opiate of the Masses". According to Bierce religion is "A daughter of Hope and Fear, explaining to Ignorance the nature of the Unknowable." I recently watched the movie Expelled where many scientists also used the word "religion" as opposed to "science". So what exactly are we meaning by the word "Religion", so that we can come to a consensus on some defintions before going much further. So how would you define the term "Religion"?