A "serious breach" refers to a significant violation or a severe failure to follow rules, standards, or agreements. It implies that the breach has serious consequences and may lead to significant damage, harm, or negative impact. Full definition
This bank is obviously collapsing and any money manager who holds onto this stock for clients is in serious breach of fiduciary duty. (investmentresearchdynamics.com)
So what does this have to do with the proposal to introduce new offences for serious breaches of privacy? (newlawjournal.co.uk)
But it did find evidence of a far more serious breach of its systems two years earlier. (nytimes.com)