The Equality Act requires you to show that you have a physical or mental impairment, which has a substantial and long-term adverse effect on your ability to carry out normal day-to-day activities. The government has published this guidance on the definition of 'disability' (PDF, 789KB).
There is also advice and guidance provided by the Equality and Human Rights Commission (EHRC).
Individual physical impairments are not listed in the Equality Act. Instead, you must have been able to satisfy the statutory test for disability, on the date that you suffered the less favourable treatment.
Long-term means the disability must have lasted, or can be expected to last, at least 12 months. People with a fluctuating condition that is likely to recur within 12 months are covered.
Some progressive conditions, for example, cancer or multiple sclerosis, are included from the moment of diagnosis.