If I rent out a room in my own home to one tenant for one year on a regular basis, would it be a Qualified Business Income? What would define it as such?

It depends.  Rental properties are typically treated as passive activities, which are excluded from the definition of a qualified trade or business. However, rentals that qualify as trades or businesses under IRC § 162 are not considered passive, and that means their income can qualify for the QBI deduction.

Here is a summary of the recent guidance given by the IRS related to Rental Properties and the QBI deduction:

https://ttlc.intuit.com/replies/7122193