"can't be defined" is correct and usable in written English.
You can use it when you want to express that something cannot be described or assigned a specific meaning. For example, "The extent of the influence the Internet has had on our lives can't be defined."
"You can't be defined by what others say about you".
Its duration can't be defined until it's completed, but by then both syllables are gone.
"The widespread problem can't be defined until we know what all the issues are".
The movie wants to show Michael as someone who needs to follow his own path, who can't be defined by labels.
Now, Mr. Slimane's clothes have it -- an erotic undercurrent that can't be defined as gay or straight, masculine or feminine, but which is there in every thought-out gesture.
The world's best dog can't be defined by a breed; it is any dog that, when you come home tired and flop onto the couch, will walk backwards and forwards under you so that your overhanging hand can stroke it without you even moving a muscle.
In contrast to CA-125, a "normal value" could not be defined for TIMP-1 and VEGF-165 and different cut-off values have been used in previous studies [ 18, 19, 25, 27- 41, 47, 53- 56].
What the film is really saying is that our cultural identity can't be easily defined.
"The private and public sphere can't be neatly defined," said Michael J. Klarman, a professor at the University of Virginia Law School.
Unlike happiness or clear air, privacy is now elevated to the status of human right, even though it can't be properly defined or nailed-down.
Consequently, the BIM uses for designing drainage system can't be entirely defined.
