Cardinal symptoms are the main features that define a condition and that a clinician looks for first when deciding whether a diagnosis fits.
For ADHD, the cardinal features are inattention, hyperactivity and impulsivity. For autism, they are differences in social communication and interaction, plus restricted or repetitive patterns of behaviour, interests or activities, including sensory differences.
We'd describe these as traits rather than symptoms, because they describe how a mind works rather than something going wrong. The manuals use 'symptoms', so assessment reports do too.
Cardinal features alone are not enough for a diagnosis. The clinician also needs to see that they've been present since childhood, show up in more than one area of life and have a real effect on daily life. Losing your keys occasionally is not the same thing.