The definition of domestic abuse can be found in the Domestic Abuse Act 2021, available in the link below:
Domestic Abuse Act 2021 - legislation.gov.uk
Behaviour of a person (“A”) towards another person (“B”) is “domestic abuse” if –
A and B are each aged 16 or over and are personally connected to each other, and the behaviour is abusive.
It does not matter whether the behaviour consists of a single incident or a course of conduct.
Children are also recognised as victims of domestic abuse under the Act:
This section applies where behaviour of a person (“A”) towards another person (“B”) is domestic abuse.
Any reference in this Act to a victim of domestic abuse includes a reference to a child who— (a) sees or hears, or experiences the effects of, the abuse, and (b) is related to A or B.
A child is related to a person for the purposes of subsection (2) if— (a) the person is a parent of, or has parental responsibility for, the child, or (b) the child and the person are relatives.
A “child” means a person under the age of 18 years.
It is worth noting that where a child is abused directly by someone personally connected to them and they are under the age of 16 (and therefore outside of the domestic abuse statutory definition), this would be considered and dealt with as child abuse.
Report a child protection concern | Gloucestershire County Council.