Demonetisation
Also called demonetization · getting demonetised · monetisation eligibility
The loss, or denial, of a channel's ability to earn ad revenue, usually following a platform review of whether its content meets monetisation policy.
In more detail
Demonetisation can mean losing monetisation on an already-approved channel, or being turned down for it in the first place, and both routes usually cite the same category of concern: whether the content is original and authentic enough to carry advertising, rather than a rule against any specific production method. Most demonetisation stories involve channels that have already built a meaningful audience and have something to lose, which is a different risk profile than a channel with a handful of videos and no subscribers.
Example
A brand-new channel publishing its first few videos is not the audience YouTube's demonetisation reviews are typically triggered by; the accounts most exposed to that risk are the ones that have already grown large enough for a policy change or a review to matter financially.
Why it matters
Nobody outside YouTube knows exactly how every demonetisation decision gets made, which makes it a poor reason to avoid starting a channel at all. The risk becomes real only once a channel is big enough for it to be worth worrying about, and reaching that point is itself a good problem to have.