What is the word/phrase for a person at a company that has very specific knowledge and nobody else at the company does?

This is a bad thing — that is, the knowledge is concentrated to one person.

I want to say it is tribal knowledge, but those words aren't correct.

It is definitely _______ knowledge.

I just can't think of the word that goes in that blank!

There are any number of synonyms you’ll find for this idea, including such words as sequestered, restricted, secluded, isolated, compartmentalized, firewalled, hidden, squirrelled-away, and walled-off.

But if you’re playing Buzzword Bingo™ for Annoying Business Jargon^® at this year’s office Christmas party, the word you’re probably looking for is siloed. Oxford Dictionaries Online says the verb means to

Isolate (one system, process, department, etc.) from others.

They provide many examples, the first two of which are:

• ‘most companies have expensive IT systems they have developed over the years, but they are siloed’
• ‘why are so many companies still siloing their SEO and social media marketing

The thing to remember with using silo and siloed this way is that this is a trendy bit of business jargon that business people use to make them seem hip and with it, all because they think it’s terminology that’s all the rage right now — without realizing they may risk coming off as pretentious and vapid to people who aren’t into that sort of ephemeral bingo-lingo.

That employee is often referred to as a subject-matter expert (SME), or generally, a pinpoint specialist or expert. They possess unique knowledge or experience in a given topic or task that makes them the sole (or primary) source of information or point of performance, or single-point-of-failure, or the go-to, on that topic.

This condition is great for job security of that individual if the unique expertise is in a productive function for the company and effectively moves the company forward, but as others pointed out it isolates talent in one individual — what happens if the SME is not able to come to work? A talent/knowledge vacuum is created as talent redundancy has been overlooked and nobody else knows how to do Sara's job, then that leg of the company experiences failure until a solution for Sara's absence is resolved.