In the context of an employment tribunal claim for disability discrimination, 'disability' has a strict legal meaning, which you can find in the Equality Act 2010.
Disability is defined as "a physical or mental impairment which has a substantial and long-term adverse effect on the ability to carry out normal day-to-day activities."
It is up to the claimant to demonstrate that they have a disability. The government has published updated guidance on the meaning of 'disability' (PDF, 789KB).