The definition of disabled the @ONS uses is: “disabled” a person who has a physical or mental health condition or illness that has lasted or is expected to last 12 months or more that reduces their ability to carry out day-to-day activities.

But in their latest report - stats covered by @DrFrancesRyan in @guardian - they used a slightly different definition.

"To define disability in this publication, we refer to the self-reported answers to the 2011 Census question, “Are your day-to-day activities limited because of a health problem or disability which has lasted, or is expected to last, at least 12 months? [...]

Include problems related to old age” (Yes, limited a lot or yes, limited a little or no). The limited a little and limited a lot categories are referred to in this article as “less-disabled” and “more-disabled” respectively, whereas people reporting no limitation on their [...]

activities are referred to as “non-disabled”. The distinction between less-disabled and more-disabled is based solely on 2011 Census data and not inferred from any other information. Therefore, it only implies a difference based on self-reported activity restrictions."

The @ONS used a separate definition for learning disability:

"Learning disability is identified from clinical diagnoses made in primary care, according to the same definition as that used in the QCOVID risk prediction model."

Disability & illnesses are often merged in official statistics

@Tanni_GT highlighted that not every disabled person has an illness.

And not everyone with a chronic illness e.g. asthma, ischemic heart disease, COPD, will identify as disabled (but they can under the Equality Act)