All of the traits of ASD also occur in people who don't have ASD – people with other disorders, people with broad autism phenotype (BAP), and just "neurotypical" people in general.
Kamp-Becker I, Albertowski K, Becker J, Ghahreman M, Langmann A, Mingebach T, Poustka L, Weber L, Schmidt H, Smidt J, Stehr T, Roessner V, Kucharczyk K, Wolff N, Stroth S. Diagnostic accuracy of the ADOS and ADOS-2 in clinical practice. Eur Child Adolesc Psychiatry. 2018 Sep;27(9):1193-1207. doi: 10.1007/s00787-018-1143-y. Epub 2018 Mar 20. PMID: 29560529:
> There is an obvious symptom overlap between ASD and emotional and anxiety disorders documented by several studies [24, 52–56]. Both disorders involve profound social interaction and communication defcits, problems in emotion recognition, insistence on sameness, infexible adherence to routines, or ritualized patterns or verbal/non-verbal behaviour. 30% of children with an anxiety disorder (but no known diagnosis of ASD) were above the cutof of the ADI-R in at least one domain [57]. Other disorders are likewise associated with “autistic traits” of profound amount, such as ADHD, conduct disorders, intellectual impairment, and language disorders (see above)
Since none of the traits of ASD is unique to ASD, none of those traits can be said to be "by definition, autism". One only should be diagnosed with ASD if one has enough of a broad enough cross-section of those traits, and those traits cause clinically significant dysfunction. (A person with a broad cross-section of those traits but without clinically significant dysfunction doesn't have ASD, they have BAP, but the boundary between ASD and BAP is unclear and varies from clinician to clinician.)
Kamp-Becker I, Albertowski K, Becker J, Ghahreman M, Langmann A, Mingebach T, Poustka L, Weber L, Schmidt H, Smidt J, Stehr T, Roessner V, Kucharczyk K, Wolff N, Stroth S. Diagnostic accuracy of the ADOS and ADOS-2 in clinical practice. Eur Child Adolesc Psychiatry. 2018 Sep;27(9):1193-1207. doi: 10.1007/s00787-018-1143-y. Epub 2018 Mar 20. PMID: 29560529:
> There is an obvious symptom overlap between ASD and emotional and anxiety disorders documented by several studies [24, 52–56]. Both disorders involve profound social interaction and communication defcits, problems in emotion recognition, insistence on sameness, infexible adherence to routines, or ritualized patterns or verbal/non-verbal behaviour. 30% of children with an anxiety disorder (but no known diagnosis of ASD) were above the cutof of the ADI-R in at least one domain [57]. Other disorders are likewise associated with “autistic traits” of profound amount, such as ADHD, conduct disorders, intellectual impairment, and language disorders (see above)
Since none of the traits of ASD is unique to ASD, none of those traits can be said to be "by definition, autism". One only should be diagnosed with ASD if one has enough of a broad enough cross-section of those traits, and those traits cause clinically significant dysfunction. (A person with a broad cross-section of those traits but without clinically significant dysfunction doesn't have ASD, they have BAP, but the boundary between ASD and BAP is unclear and varies from clinician to clinician.)