Dyscalculia is a specific learning difficulty that affects the ability to understand numbers and perform mathematics. Although there are many people who found maths challenging at school, dyscalculia is a more profound difficulty that leads to a lack of 'number sense', the ability to understand what the word 'five' really means, for example.
Research into dyscalculia is roughly 30 years behind dyslexia and the British Dyslexia Association has only recently published their official definition:
Dyscalculia is a specific and persistent difficulty in understanding numbers which can lead to a diverse range of difficulties with mathematics. It will be unexpected in relation to age, level of education and experience and occurs across all ages and abilities.
Mathematics difficulties are best thought of as a continuum, not a distinct category, and they have many causal factors. Dyscalculia falls at one end of the spectrum and will be distinguishable from other maths issues due to the severity of difficulties with number sense, including subitising, symbolic and non-symbolic magnitude comparison, and ordering. It can occur singly but often co-occurs with other specific learning difficulties, mathematics anxiety and medical conditions.
(P Jarrett 2019)