Earlier it gave the indefinite form "et museum". So this word is irregular, you can't say "museumet"?
Neuter '-ium'- and 'eum'-nouns have the 'um'-part dropped in the definite case.
et laboratorium - laboratoriet
et studium - studiet
According to the grammar book I own, this is the case only for neuter nouns with "-ium" and "-eum" at the end. Other neuter nouns with "-um" ending (eg. datum, forum) don't drop the "-um" part in def.sing. form.
I may have oversimplified a bit too much :)
Even your rule has its flaws, for instance 'barium' and 'gallium' breaks it, so it's probably better to learn them case by case.
I also think it's better to learn them one by one rather than trying to memorise tons of rules and exceptions. :)