Phenomenal concepts are concepts which one forms when introspecting. They describe what conscious experience appears to be, how conscious experience presents itself to the introspecting subject.
It is important to distinguish phenomenal concepts from phenomenal consciousness and similar concepts. Models of consciousness generally seem to try to model phenomenal concepts. According to illusionism or eliminativism, the relation between phenomenal concepts and phenomenal consciousness is non-trivial and complex, so that it would be a mistake to take phenomenal concepts as guiding the construction of theories of consciousness.
In several publications, arguments are advanced which aim to show that special features of phenomenal concepts (which other concepts do not possess) are responsible for the illusion of there being a hard problem or explanatory gap.