Equality Act 2010
You are regarded as disabled under the act if you have a physical or mental impairment that has a ‘substantial‘ and ‘long term” negative effect on your ability to do normal daily activities.
Substantial
More than minor or trivial; for example, it takes you much longer than it usually would to complete a daily task like getting dressed.
Long term
Lasting twelve months or more; for example, a chronic breathing condition that develops as a result of an infection.
The General Medical Council discuss the definition on their website.