The product of Einstein's most famous equation (6)
I believe the answer is:
energy
(Other definitions for energy that I've seen before include "Pep" , "life" , "Vitality, get-up-and-go" , "Vivacity" , "Vigour, spirit" .)
I believe the answer is:
energy
(Other definitions for energy that I've seen before include "Pep" , "life" , "Vitality, get-up-and-go" , "Vivacity" , "Vigour, spirit" .)