Bose-Einstein Condensates. What is a "matter wave"? Greetings. I was reading a rather non-descriptive page about Bose-Einstein Condensates. I've never really read much about them before, but now I find myself interested. This page mentioned that BECs form "matter waves" about a millimetre across. - What is a "matter wave"? - How exactly do you decide that a chunk of matter is a wave? In which ways does this matter "behave as a wave"?