What is the PROPER definition of "Heavy Metal"?

I'm pretty sure that Black Sabbath, Ozzy, Motorhead, and Megadeth are Heavy Metal, but Blue Oyster Cult!!!??? Aerosmith!!!??? Led Zeppelin!!!??? NO!!!!!!! I'm pretty sure they are classified as classic rock or soft rock. Nowhere are they even remotely close to being Heavy Metal... So, my friends, what is the PROPER definition of Heavy Metal????