I've seen the term 'by mass' being used in various sorts of contexts. Some, while explaining the law of definite proportions say, the 'same proportions by mass', some say 'constant chemical composition by mass'.
What does the term 'by mass' mean? Does it mean in a proportion/ratio, we have mass as both, the denominator and numerator?
When I say 'percentage by mass', what would that mean?