Dyslexia
is quite problematic concept with many different definitions.
Research Definition used by the National Institutes of Health:
Dyslexia is a specific learning disability that is neurological in
origin. It is characterized by difficulties with accurate and/or
fluent word recognition, and by poor spelling and decoding abilities.

Because
of the different ways to define or operationalize “dyslexia” or
“reading disability” it is impossible to know exact prevalence.
Although much bigger amount of children struggle with reading
(especially in developing countries), the strict estimates vary from
4-5%.

These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.

During the last years many researchers (see e.g. Elliott & Grigorenko, 2014) have suggested that “reading disability” could be more neutral and useful concept to describe those children who struggle with reading,

In
eTALE Africa web-pages we prefer to use term “reading difficulty”
and “reading disability” instead of “dyslexia”.