Limited literacy presents a significant challenge in HCI research, yet the field lacks consistent definitions and measurement criteria.Researchers often interchange terms such as 'functional illiterates,' 'low literates,' and 'semi-literates,' further complicating the field. This paper conducts a systematic literature review (SLR) of 33 HCI studies, revealing concerns about the absence of a definition in 41% of the studies and the lack of measurement technique in 74%. Based on the results from our SLR and relevant research beyond HCI, we propose the following work-in-progress definition. 'Functional illiterates are motivated adults with some familiarity with text but insufficient to fully comprehend meanings and low skills in the measured digital skill, with enough language proficiency in the study language if they are literate in their native language. This understanding, coupled with addressing the identified issues, will empower the HCI4D community to design more inclusive technology solutions for functionally illiterate users in developing countries.CCS CONCEPTS •Social and professional topics~User characteristics •Human-centered computing~Accessibility~Accessibility theory, concepts and paradigms •Human-centered computing~Human computer interaction (HCI)~HCI theory, concepts and models.