AS is Asperger Syndrome, of course
DD is dear daughter
DS is dear son
BAP is broader autistic phenotype - in other words, a person with mild autistic traits that don't meet criteria for an autism spectrum condition. Sometimes they're also called 'cousins', particularly if they're diagnosed with a disability other than autism (eg ADHD)
AC is autistics and cousins - cousins defined under BAP
ADHD/ADD is Attention Deficit Hyperactivity Disorder or Attention Deficit Disorder - ADD is the older term, also sometimes used to refer to the inattentive subtype of ADHD (who lack hyperactivity)
Any others?
This one should probably be stickied, by the way.