Greg Esres wrote

"Solo instruction" seems an oxymoron. The term doesn't appear
in the FARs, nor in the FAQs.

Perhaps I should have said "solo flight training". :-)

Section 61.109: Aeronautical experience.
(a) For an airplane single-engine rating. Except as provided in
paragraph (i) of this section, a person who applies for a private
pilot certificate with an airplane category and single-engine class
rating must log at least 40 hours of flight time that includes at
least 20 hours of flight training from an authorized instructor and
10 hours of solo flight training in the areas of operation listed in
§61.107(b)(1) of this part, and the training must include at least --

Bob Moore