In standard English, a tenet is a fundamental belief held by a group of people. At Microsoft, the term tenet is used as a generalization of what we previously called taxes: Things that everybody has to do in order to be a good software citizen.
While taxes are typically very low-level and specific, like supporting roaming user profiles or multiple monitors, tenets are broader concepts like reliability or compatibility or API design and documentation.
The word doesn't escape the halls of Microsoft very often. Most of the citations I have are internal, but here's a case where Jon DeVaan let it slip into a keynote address:
And together we've improved reliability on Vista huge amount. And when you look at the compatibility tenet that we have for Windows 7, all of this work translates immediately to the reliability of Windows 7 on day one.
Occasionally, somebody will confuse the word tenet with the superficially-similar tenant.
We still have many bugs marked as High tenant impact.
- tenant: Somebody who is renting from you.
- tenet: An important principle or belief.
Bad pun alert: I guess a bug could be high tenant impact if it's a bedbug or lice.