Unlawful discrimination is unfair treatment received by an individual because of a characteristic they possess.
These are called 'protected characteristics' in the Equality Act 2010 which is the main piece of anti-discrimination legislation in the UK.
The protected characteristics are:
The unfair treatment that is received can be of a number of types where disadvantage is caused directly, indirectly or by harassment of some sort. The Equality Act 2010 calls these types of treatment 'prohibited conduct'.