The New Definition of Dyslexia: What Has Changed and What Does It Mean?

Deborah Woolley

3 hours ago

4 min read

A new definition of dyslexia was published following the Dyslexia Delphi Study. The study brought together expert researchers and professionals to develop an evidence-informed consensus about how dyslexia should be understood and described.

For anyone considering a dyslexia assessment for themselves or their child, this does not mean that dyslexia has suddenly become something different. Rather, the new definition reflects developments in research and our understanding of the different factors that can contribute to an individual's dyslexic profile.

The 2025 Delphi Definition of Dyslexia

The definition states:

Delphi definition of dyslexia (February 2025)

· “Dyslexia is a set of processing difficulties that affect the acquisition of reading and spelling.

· In dyslexia, some or all aspects of literacy attainment are weak in relation to age, standard teaching and instruction, and level of other attainments.

· Across languages and age groups, difficulties in reading fluency and spelling are a key marker of dyslexia.

· The nature and developmental trajectory of dyslexia depends on multiple genetic and environmental influences.

· Dyslexia can affect the acquisition of other skills, such as mathematics, reading comprehension or learning another language.

· The most commonly observed cognitive impairment in dyslexia is a difficulty with phonological processing (i.e. in phonological awareness, phonological processing speed or phonological memory). However, phonological difficulties do not fully explain the variability that is observed.

· Working memory, processing speed, and orthographic skills can contribute to the impact of dyslexia.

· Dyslexia frequently co-occurs with one or more other developmental difficulties, including developmental language disorder, dyscalculia, ADHD, and developmental co-ordination disorder.”

What does this mean in practice?

One of the important messages within the new definition is that dyslexia cannot be understood by looking at one skill or one test score in isolation. Reading fluency and spelling are identified as key markers of dyslexia, but the definition also recognises the range of processing difficulties that may contribute to an individual's profile.

Phonological processing remains important

Phonological processing refers to the ability to recognise, remember and work with the sounds within language. Difficulties in this area remain the most commonly observed cognitive impairment associated with dyslexia. However, the new definition makes it clear that phonological difficulties do not explain every dyslexic profile.

Other processing skills can contribute

Working memory, processing speed and orthographic skills are also recognised as factors that can contribute to the impact of dyslexia. This is important because dyslexia does not look exactly the same in everyone. One person may have significant difficulties with spelling and phonological processing, while another may read accurately but lack fluency, require additional time to process written information, or find it difficult to retain and manipulate information while completing a task.

Reading fluency and spelling are important

The definition specifically identifies difficulties with reading fluency and spelling as key markers of dyslexia across languages and age groups. This is particularly important when considering individuals who have developed good reading accuracy. Being able to read words correctly does not necessarily mean that reading is effortless or efficient. Some individuals may read accurately but slowly, need to reread information, or expend considerably more effort than their peers.

Dyslexia develops within a wider context

The new definition recognises that the nature and developmental trajectory of dyslexia are influenced by multiple genetic and environmental factors. Teaching, intervention, educational experiences and the strategies an individual develops over time can all influence how difficulties present. This helps explain why dyslexia may look different at different stages of a person's education or working life and why some individuals become very effective at compensating for their difficulties.

Dyslexia can affect more than reading and spelling

Although reading and spelling are central to dyslexia, its effects can extend further.

The definition recognises that dyslexia can affect the acquisition of other skills, including mathematics, reading comprehension and learning another language. The impact will vary considerably between individuals, which is why it is important to understand a person's particular pattern of strengths and difficulties.

Dyslexia can co-occur with other developmental difficulties

The definition also explicitly recognises that dyslexia frequently occurs alongside other developmental difficulties, including developmental language disorder, dyscalculia, ADHD and developmental co-ordination disorder (DCD). This does not mean that a person with dyslexia will necessarily have another developmental difficulty. It does, however, reinforce the importance of considering the whole individual profile during assessment.

What does the new definition mean for a dyslexia assessment?

A diagnostic assessment is therefore much more than looking for a particular low score or identifying one underlying processing weakness. Assessment involves bringing together evidence from an individual's developmental and educational history, current experiences and difficulties, literacy attainment, cognitive processing and wider pattern of strengths and weaknesses.

As a Specialist Teacher Assessor, I use current professional guidance and the 2025 Delphi Definition of Dyslexia when carrying out diagnostic assessments. The purpose of an assessment is not simply to establish whether an individual meets the criteria for dyslexia, but also to develop a clear understanding of their individual learning profile and how this affects them in education, employment and everyday life. This understanding can then inform appropriate recommendations, strategies and support.