Writing about an opt-out testing movement brewing in her children's elementary school (and elsewhere), New Yorker writer Rebecca Mead used the highly-specific invigilate when she wrote: 

Teachers invigilating the exams were shocked by ambiguous test questions, based, as they saw it, on false premises and wrongheaded educational principles.

That invigilate's so rare may seem surprising given that it describes a phenomenon taking place in thousands of schools and classrooms around the world at every minute of every day. The word means "watch over students taking a test to prevent cheating." (You morphology fans had probably already inferred that it had something to do with "watching over" as the Latin vigilare means just that.) 

So why is invigilate so unusual? First, there's a British English/American English divide, with Americans more likely to use proctor. (See Separated by a Common Language's take on this here.) But maybe the fault also lies with the word's specificity. Like a single-purpose kitchen tool (think: melon baller) assigned to the trash heap while a multipurpose tool that performs the same task less well (think: knife) continues to earn its spot in the drawer, invigilate is jettisoned in favor of more general synonyms proctor and monitor.

A quick look at the frequency-checker located on every word's Vocabulary.com Dictionary definition pages show us how far up on the endangered species list invigilate really is. Compare invigilate with test, which appears once in every 69 pages, monitor, which appears once every 1,807 pages, and proctor, which you can expect to encounter every 3,198 pages.

You can expect to see invigilate once in every 11,819,311 pages of text. That's nearly 3,700 times less often than you'll see proctor. Or to put it in different terms: You could read 8,207 novels the length of War and Peace before you could expect to encounter this word.

But, hey, you just saw it here. Which means today's your day for long odds. Go out and buy a lottery ticket.