Comorbidity means two or more conditions present in the same person at the same time, such as ADHD and anxiety, or autism and ADHD together.
Many clinicians now say co-occurring conditions instead. It describes the same thing in plainer words.
Co-occurring conditions are common with ADHD and autism. ADHD and autism often occur together, which is called AuDHD. Anxiety, depression, sleep problems, dyslexia and tic disorders also occur alongside them more often than in the general population.
This matters for assessment because one condition can hide another. Treating anxiety without noticing the ADHD can leave someone with only part of an answer. A good assessment looks at all of your history rather than stopping at the first diagnosis that fits.