Anhedonia: the inability to experience pleasure from activities usually found enjoyable.
Although by no means an expert on anhedonia (or much else), I believe that it is a concept quite important when it comes to making a diagnosis of depression although the precise hows and whys of connecting anhedonia to other symptoms are still the source of some discussion .
The paper from Vicki Bitsika & Christopher Sharpley  brings the concepts of anhedonia and depression into view with autism in mind. Based on the analysis of self-reported symptoms on the "Depression subscale of the Child and Adolescent Symptoms Inventory (CASI-D)" for 70 males diagnosed with an autism spectrum disorder (ASD) and 50 asymptomatic controls matched for age, authors reported that: "The MDD [major depressive disorder] profiles for the ASD participants were dominated by anhedonia."
Accepting that the label of autism seemingly very rarely exists in a diagnostic vacuum (see here) and that there is still some debate about whether comorbidity is just that or something rather more integral to parts of the growing pluralisation of autism (see here), the Bitsika/Sharpley paper is a potentially important one. Getting into the nitty-gritty details of how issues such as depression manifest on top of a diagnosis of autism is important as per the discussions by Vannucchi et al  (see here) on the "atypical presentation" of bipolar disorder with Asperger syndrome in mind. Knowing for example, that anhedonia might be more characteristic of MDD in cases of autism or even more centrally to autism , may offer not only a more detailed perspective on screening for and managing such issues as and when they occur, but also a little bit more detail about the mechanisms through which such symptoms may come about.
Music: Mercury Rev - Goddess on a Highway.
 Gaillard R. et al. Anhedonia in depression. Encephale. 2013 Sep;39(4):296-305.
 Bitsika V. & Sharpley CF. Differences in the Prevalence, Severity and Symptom Profiles of Depression in Boys and Adolescents with an Autism Spectrum Disorder versus Normally Developing Controls. International Journal of Disability, Development and Education. 2015; 62: 158-167.
 Vannucchi G. et al. Bipolar disorder in adults with Asperger׳s Syndrome: A systematic review. J Affect Disord. 2014 Jul 8;168C:151-160.
 Chevallier C. et al. Brief report: Selective social anhedonia in high functioning autism. J Autism Dev Disord. 2012 Jul;42(7):1504-9.
Bitsika, V., & Sharpley, C. (2015). Differences in the Prevalence, Severity and Symptom Profiles of Depression in Boys and Adolescents with an Autism Spectrum Disorder versus Normally Developing Controls International Journal of Disability, Development and Education, 62 (2), 158-167 DOI: 10.1080/1034912X.2014.998179