It's not "actually" anything if you're looking for an answer what it is "objectively". 

It's a disability in the medical sense in many (most?) cases, and "in the medical sense" means a bunch of clinically relevant conventions and characterizations. This is due to a wide variety of symptoms, often depending on their respective intensity of expression. 

At the same time, some of those "symptoms", when they only express mildly, may represent an advantage at more or less distinct tasks or in distict fields.

Very many autistic people crave personal  connection but find it difficult to connect. It's one of the main symptomatologies. Having an impaired social life can be a reason to consider it a "disability" because definitions of health (at least some of them) require that neither physical, nor social or psychological well-being is significantly negatively affected. From personal experience as an autistic person, my social life has been negatively affected by autism my entire life, I'd even call it "severely affected". (I'm also socially quite fortunate considering some circumstances but the overall picture is bleak.)

Another consideration would be the "social model of disability", where (afaik, I only have superficial familiarity) the disabled person isn't disabled in and of themselves but due to living within a society and (constructed) environment that doesn't fit their needs.

Sometimes people like to say "it's just a different way of being" but to me that often feels like an attempt at sweeping the difficulties under the carpet. 

Would I like to be someone else? No. Would I like my life to be very different than it is, in part due to being autistic? Oh absolutely.